Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Removing and recycling removed material from an ongoing...
Reexamination Certificate
2001-08-09
2002-10-08
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Removing and recycling removed material from an ongoing...
C526S348000, C526S071000, C526S082000, C422S139000, C422S171000, C422S198000
Reexamination Certificate
active
06462150
ABSTRACT:
The present invention relates to a method for the isolation of olefins from polyolefin plants, and to a method and an apparatus for the preparation of polyolefins.
BACKGROUND OF THE INVENTION
Gas-phase polymerization is an important technology for the preparation of polyolefins. the catalyst, which is usually employed in supported form, is in the form of small, free-flowing particles which serve as starting points for the polymerization. In general, gas-phase polymerization is carried out as a fluidizedbed process. In this, the catalyst particles are in the form of a fluidized bed during the polymerization. The monomers are generally introduced with the carrier gas into the corresponding polyolefin plant, which is preferably designed as a fluidized-bed polymerization reactor. Catalyst particles are added elsewhere in the polymerization reactor, usually together with inert gas. Particularly suitable catalysts are Philipps (based on chromium/silica gel), Ziegler or metallocene catalysts. The addition of inert gas is necessary since the basic heterogeneously catalyzed polymerization is usually moisture- and/or oxygen-sensitive. The resultant polymer is produced in solid form, which means that the catalyst and polymer exist alongside one another in the fluidized bed. Unreacted monomer is usually circulated thereby dissipating the heat of the reaction.
Gas-phase polymerizations are used, for example, to prepare polyethylene or polypropylene. However, this process can also be used to prepare polymers which are composed of a plurality of different monomer units, so-called copolymers. Suitable catalysts for the gas-phase polymerization are, as described in the introduction, in particular Ziegler, Philipps and metallocene catalysts. Philipps and metallocene catalysts are very sensitive to catalyst poisons, which may suppress the polymerization even in low concentrations (in the ppm range). Catalyst poisons of this type are, for example, sulfur compounds, such as sulfur dioxide or hydrogen sulfide. These frequently occur in such small concentrations that they cannot be detected directly from the gaseous reaction mixture. It is therefore frequently difficult to judge which catalyst poisons are responsible for the inactivity of a catalyst. Since catalyst poisons generally accumulate during continuous operation of gas-phase polymerization plants, it is necessary to remove some of the gaseous reaction mixture from the reactor as offgas during operation.
The disadvantage exists here that olefins are also lost as valuable starting materials if the offgas is not worked up. The offgas in gas-phase polymerizations generally comprises predominantly inert gas and unreacted olefin. If inert gas is not partially removed from the reactor, it accumulates, since it is not reacted in the polymerization. For this reason, it is necessary to continuously remove a constant offgas stream from the reactor in continuous operation. In general, valuable olefin is removed together with inert gas and other secondary components, such as, for example, catalyst poisons, the olefin present in the gas mixture removed is subsequently burnt, and the resultant combustion gas is finally discarded. The loss of olefin is financially significant, and it has consequently been attempted to recover the olefin and to recycle it into the polymerization reactor.
The isolation of the polyolefin from the offgas can be achieved, for example, in accordance with U.S. Pat. No. 5,521,264, by extraction of the olefin, it being necessary to separate the olefin from the extractant in a next step. This separation is quite complex in equipment terms, which means that it is generally more economic and inexpensive not to separate off the olefin and to discard it (after combustion) with the offgas.
SUMMARY OF THE INVENTION
The present invention has the object of improving the generic method for the preparation of polyolefins in such a way that the polyolefins present in the offgas can be recovered and subsequently fed back into the polymerization reactor. It is of particular importance here that the recovered olefin is freed both from inert gases and from by-products, in particular catalyst poisons. The equipment complexity should be kept as low as possible here, so that the recovery of the olefins is economically viable.
This object is achieved by a method for the isolation of olefins from a gas mixture comprising one or more olefins, inert gas and catalyst poison, in which the gas mixture is fed to a separation unit and separated into olefin and inert gas. The method according to the invention is then characterized in that an apparatus is connected upstream or downstream of the separation unit and that
i) in the case of an upstream apparatus, the catalyst poison is removed at least partially from the gas mixture, and
ii) in the case of a downstream apparatus, the catalyst poison is removed at least partially from the separated-off olefin.
According to a preferred embodiment of the invention, the gas mixture is produced as offgas in the catalytic gas-phase polymerization of olefins.
The method according to the invention is particularly suitable if metallocene or chromium/silica gel catalysts are used for the polymerization, since these catalysts are particularly sensitive to catalyst poisons. These catalyst poisons are frequently virtually impossible to identify, which means that it is of great importance to remove even extremely small amounts of impurities, even in the ppm range, as completely as possible. In general, the proportion of the catalyst poison in the gas mixture is less than 10
−3
% by weight.
An apparatus which is particularly suitable for removing the catalyst poison in accordance with the invention is one which is designed for mass separation. The term apparatus for mass separation is intended to mean equipment which contains one or more molecular sieves which are able to adsorb, absorb or chemically bind catalyst poison. The equipment can contain, for example, molecular sieve such as activated carbon, zeolites or washing liquid. A suitable corresponding adsorption method is, in particular, pressure sowing adsorption (PSA). However, also suitable in principle are apparatuses with the aid of which catalyst poisons are chemically reacted, with the corresponding secondary products of the catalyst poisons themselves no longer being able to act as catalyst poison—apparatuses of this type can then release these secondary products again (into the gas stream). A condensation device which operates on the principle of low-temperature separation, is likewise a suitable apparatus, with, in this embodiment, the catalyst poison being removed by fractional condensation.
In the operating variant of the mass separation apparatus by the adsorption method which is particularly preferred according to the invention, the operating temperature in the loading phase is in the range from −30 to 100° C., preferably in the range from 10 to 50° C., while the pressure in this phase is in the range from 3 to 50 bar, preferably from 10 to 30 bar. In the case of operation with periodic change of the adsorbent or with periodic regeneration of the adsorbent, for example by means of nitrogen, the optimum temperatures for operation of the apparatus are, in accordance with the invention, in the range from 80 to 240° C.
Generally the offgas formed in the polymerization process essentially consists (preferably to the extent of greater than 95%) of inert gas and olefin. This gas mixture generally comprises from 20 to 80% by volume, preferably from 40 to 60% by volume, of olefin. The isolation of the olefin from the inert gas is carried out using a separation unit which is, for example, a membrane device which contains one or more membranes, or a distillation device. Membrane devices of this type for the recovery of olefins usually contain one or more diffusion membranes which retain inert gases, such as, for example, nitrogen, and are permeable to relevant olefins (generally to many organic compounds, in particular to numerous hydrocarbons) [M. Jacobs,
Bitterlich Stefan
Evertz Kaspar
Feindt Hans-Jacob
Hecker Manfred
Basell Polyolefine GmbH
Cheung William K
Lydon James C.
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