Patent Application
20080300417
The present invention relates to an improved slurry reaction system such as that employed in the production of propylene oxide by reaction of oxygen, hydrogen and propylene in a liquid reaction medium such as a water and methanol mixture and employing a solid catalyst such as palladium promoted TS-1 slurried therein, and especially to a system wherein the reaction liquid slurry is first subjected to a separation treatment for removal of solid catalyst particles of relatively large size for recycle from liquid which contains reaction product as well as catalyst fines. The mixture containing product and fines is then subjected to further separation treatment for solid fines removal.
Epoxides constitute an important class of chemical intermediates useful for the preparation of polyether polyols, glycols, glycol ethers, surfactants, functional fluids, fuel additives and the like. Many different methods for synthesizing epoxides from the corresponding olefins have been described in the literature. A Japanese patent application assigned to the Tosoh Corporation and published in 1992 (Kokai No. 4-352771) proposed making propylene oxide by reacting propylene, hydrogen and oxygen using a catalyst comprising a Group VIII metal and a crystalline titanosilicate. U.S. Pat. Nos. 6,281,369, 6,005,123 and 6,008,388 are also relevant. As with any chemical process, it would be desirable to attain further improvements in epoxidation methods of this type and it is to such improvements what this invention is directed.
A problem with prior systems has been the tendency for the slurried solid catalyst to undergo attrition due to agitation either by impellers and/or gases. The continuous generation and accumulation of fines within the slurry reactor causes problems in the separation of solids from liquids by filtration as the catalyst fines tend to plug or otherwise interfere with the separation procedures.
It is important in the slurry systems that the solid catalyst have a suitable size to function properly. Generally catalyst particles having an effective diameter of 10 to 100 microns, preferably 30 to 40 microns are suitable. Particles having an effective diameter below about 1 micron interfere with system operation.
In accordance with the present invention, the slurry reaction mixture is subjected to a plurality of separation steps whereby the slurry is first treated to separate the relatively large size catalyst particles which are useful in the system from reaction liquid which also contains catalyst fines. The separated larger sized particles can conveniently be recycled to the reaction. The mixture of reaction liquid and fines is then treated in one or more filtration steps to separate solids free reaction liquid from the solids fines.
The accompanying
Accompanying
Accompanying
Practice of the invention is especially advantageous in a system for the production of propylene oxide by reaction of propylene, oxygen and hydrogen in a liquid comprised of a solvent such as methanol or a methanol-water mixture and containing slurried noble metal promoted titanium silicalite.
The catalysts to be used in the present invention are comprised of a titanium or vanadium zeolite and a noble metal (preferably an element of Group VIII of the Periodic Table). Suitable zeolites are those crystalline materials having a porous molecular sieve structure with titanium or vanadium atoms substituted in the framework. The choice of zeolite will depend upon a number of factors, including the size and shape of the olefin to be epoxidized. For example, it is preferred to use a relatively small pore titanium or vanadium zeolite such as a titanium silicalite if the olefin is a lower aliphatic olefin such as ethylene, propylene, or 1-butene. Where the olefin is propylene, the use of a TS-1 titanium silicalite or vanadium silicalite is especially advantageous. For a bulky olefin such as cyclohexene, a larger port titanium zeolite such as a titanium zeolite having a structure isomorphous with zeolite beta may be preferred.
The titanium-containing zeolites useful as catalysts in the epoxidation step of the process comprise the class of zeolitic substances wherein titanium atoms are substituted for a portion of the silicon atoms in the lattice framework of a molecular sieve. Such substances are well known in the art.
Particularly preferred titanium-containing zeolites include the class of molecular sieves commonly referred to as titanium silicalites, particularly “TS-1” (having an MEL topology analogous to that of the ZSM-5 aluminosilicate zeolites), “TS-2” (having a MEL topology analogous to that of the ZSM-11 aluminosilicate zeolites), and “TS-3” (as described in Belgian Pat. No. 1,001,038). Also suitable for use are the titanium-containing molecular sieves having framework structures, isomorphous to zeolite beta, mordenite, ZSM-48, ZSM-12 and MCM-41. The titanium-containing zeolite preferably contains no elements other than titanium, silicon and oxygen in the lattice framework, although minor amounts of boron, iron, aluminum, and the like may be present. Other metals such as tin or vanadium may also be present in the lattice framework of the zeolite in addition to the titanium as described in U.S. Pat. Nos. 5,780,654 and 5,744,619.
Preferred titanium-containing zeolite catalysts suitable for use in the process of this invention will generally have a composition corresponding to the following empirical formula: xTiO2(1-x)(SiO2), where x is between 0.0001 and 0.500. More preferably, the value of x is from 0.01 to 0.125. The molar ratio of Si:Ti in the lattice framework of the zeolite is advantageously from 9.5:1 to 99:1 (most preferably from 9.5:1 to 60:1). The use of relatively titanium-rich zeolites may also be desirable.
While any of the noble metals can be utilized (i.e., gold, silver, platinum, palladium, iridium, ruthenium, osmium), either alone or in combination, palladium is particularly desirable. Typically, the amount of noble metal present in the catalyst will be in the range of from 0.01 to 20 weight percent, preferably 0.1 to 5 weight percent. Suitable catalysts are described in U.S. Pat. Nos. 6,281,369, 6,005,123 and 6,008,388 the disclosure of which are incorporated herein by reference.
The amount of catalyst used may be determined on the basis of the molar ratio of the titanium contained in the titanium zeolite to the olefin that is supplied per unit of time. Typically, sufficient catalyst is present to provide a feed ratio of from about 10 to 100,000 lbs of propylene/hr per lb of titanium.
Referring to
The reaction liquid and fines admixture passes via line 3 to secondary filtration units 4 which effectively filter out fines from the reaction liquid. The reaction liquid essentially free of solids passes via line 5 to tertiary filter system 6 for further removal of fines. In many cases, the tertiary filtration step can be omitted as solids separation is effectively complete with filtration means 2 and 4.
The product containing reaction liquid passes from filtration unit 6 via line 9 for further treatment and product recovery (not shown).
As shown in
In accordance with this embodiment, filter 2 is sized to effectively retain the appropriate sized catalyst particles in the reaction liquid in reactor 1 while removing liquid reaction mixture and catalyst fines. The fines are separated from reaction liquid in the filtration units 4 and 6 by conventional operation.
A somewhat different embodiment of the invention is shown in attached
Generally speaking, both the internal and external filters are sized for a liquid flow or flux of about 0.01 to about 2 gallons per minute per square foot. The filtration step is operated at a differential pressure of about 0 to about 120 psia.
In the settler the larger particles are separated and returned via line 24 to reactor 21. Reaction liquid containing catalyst fines is passed via line 25 to filters 26 and 27 for final separation of fines as described above.
In place of the settler, equivalent separating means such as hydroclones or elutriators can be used.
The following examples illustrate the invention.
Referring to
The reaction is carried out at 60° C. and 300 psig with a residence time in the reactor of 2-4 hours. In the course of reaction with agitation and gas sparging over 2 months a substantial portion of the catalyst particles are reduced in particle size by attrition to finer particles of 1 micron diameter or less at a rate of about 0.14% per day.
Means are provided in reactor 1 in the form of a slurry draw-off for continuous removal of reaction product containing liquid. Surrounding the slurry draw-off is filter screen 2 which is sized to permit passage of liquid containing the catalyst fines while retaining particles of 1 micron diameter or greater in reactor 1. Screen 2 is appropriately 5 micron or finer and is operated at a liquid flux or flow rate of 0.15 gpm/sq.ft. Filter 2 is backwashed with solvent feed when filter differential pressure reaches 80 psia.
Reaction liquid containing catalyst fines passes via line 3 to filters 4 which are operated alternately and which effectively retain catalyst fines which permitting passage of reaction liquid. The reaction liquid passes via line 5 to finishing filter 6 wherein fines separation is completed. In many cases, filter 6 is not needed as the filtration is completed in filters 4.
From filter 6, the reaction liquid passes via line 10 to product separation and solvent recycle which are carried out in accordance with normal practice.
As indicated above, solvent and make-up catalyst are added as needed via line 8.
Referring to
Reactor mixture with contained catalyst fines is passed via line 13 to filters 14 where it is treated as above described in Example 1.
Practice of the invention is especially advantageous in that continuous operation of the system is achieved without the difficulties caused by the accumulation of fines as contrasted with prior practices.
