PATENT
Multistage
process for the polymerization of olefins
United States Patent 8097686
Inventors:
Maurizio, Dorini (Mantova, IT)
Penzo, Giuseppe (Mantova, IT)
Riccardo, Rinaldi (Mantova, IT)
Application Number: 12/733773
Publication Date: 01/17/2012
Assignee: Basell Poliolefine Italia s.r.l. (Milan, IT)
Abstract:
A
process for the multistage polymerization of olefins in a sequence of an
upstream slurry reactor and a downstream gas-phase reactor, the transfer of
polymer from the upstream reactor to the downstream reactor comprising the
following steps: (a) heating the slurry of polyolefin particles to evaporate
the liquid polymerization medium; (b) separating the polyolefin particles from
the obtained gaseous phase in at least a separation chamber; (c) transferring
the polyolefin particles to said downstream reactor by means of a couple of
lock hoppers working intermittently in parallel, where one of said lock hoppers
is continuously filled with the polymer coming from said separation chamber,
while simultaneously the other one is continuously pressurized by means of a
gas comprising the reaction mixture coming from said downstream reactor.
The present invention relates to a multistage process for the
polymerisation of olefins The invention relates also to an apparatus for
carrying out such a multistage polymerization process.
The polymerization of olefins in two or more serially connected polymerization
reactors allows to produce olefin polymers with improved mechanical properties.
This is made possible by choosing polymerisation conditions in the second or
subsequent reactors different from the reaction conditions existing in the
first polymerization reactor. Typically, olefin polymers grow on granules
including a catalyst component, which continues to exert a catalytic activity
even when the polymer particles are transferred to a successive polymerization
reactor. The polymer resulting from the first polymerization reactor is
transferred to the second polymerization reactor, where polymerization is continued
under different conditions. Therefore, different fractions of polymer can grow
on the same catalytic granule by maintaining a different concentration of
monomers in each reactor.
Examples of polymers that may be produced by a multistage polymerization
process include bimodal or multimodal polymers obtained by maintaining a
different concentration of chain terminator, such as hydrogen, in each reactor;
and random or heterophasic copolymers obtained by polymerizing different
(co)monomers in each reactor. The term “heterophasic copolymer” includes also
in-reactor polymer blends.
It is also known the use of a multistage process comprising one or two slurry
polymerization reactors and one or more gas-phase reactors connected
successively to each other to produce random and/or impact copolymers of
propylene, thereby expanding and modifying some mechanical and physical
properties of conventional polypropylene products.
When propylene homopolymer is transferred from a slurry polymerization reactor
to a successive gas-phase reactor, the slurry containing polypropylene must be
necessarily heated and flashed in a flash vessel, so as to volatilize the
liquid polymerization medium and to separate the polymer from the evaporated
phase. It is clear that a direct transfer of the separated polymer from the
flash vessel to the downstream gas-phase reactor would not allow to maintain
different polymerization conditions in the downstream reactor, due to the
substantial amount of gases and dissolved hydrocarbons associated to the
polymer coming from the upstream reactor. Especially when a specific olefin
monomer (e.g. propylene) is polymerized in the upstream reactor and this
monomer is absent in the downstream reactor, it becomes important to prevent
said monomer from entering the downstream polymerization apparatus, even in
little amounts.
Some technical solutions to the above problem have been proposed in case of
olefin polymerization performed in two serially connected gas-phase reactors.
According to EP-B-192427 the transfer of polymer from an upstream gas-phase
reactor to the downstream gas-phase reactor is performed by means of a gas
stream comprising the gas mixture coming from the gas recycle line of the
downstream reactor. Said gas mixture has been cooled to a temperature of at
least 20° C. lower than the temperature of the downstream reactor. The
described transfer device comprises at least three separate vessels: a
discharge vessel connected to the upstream reactor, equipped with valves to
prevent excessive withdrawal of polymer from the upstream reactor; a
decompression chamber equipped with valves and connected to the discharge
vessel; a compression chamber equipped with valves and connected to the
downstream reactor. The path of the polymer through this series of vessels and
associated valves and pipes is rather tortuous and, in case of polymers having
tendency to be sticky or to be compacted, can generate undesired polymer
aggregates and, eventually, chunks that compromise the operation of the plant.
Above all, the above transfer device suffers the great disadvantage of failing
in providing a continuous transfer of polymer from the upstream gas-reactor
reactor to the downstream reactor, as pointed out by the disclosure of
EP-B-192427 (col. 14, lines 14-19): all the operations of withdrawal,
decompression, compression, transfer and introduction of polymer into the
downstream reactor are performed periodically.
A similar transfer device is disclosed in EP-B-050013. According to this
patent, the transfer device comprises a container where the polymer is
discharged, said container defining an inert gas zone wherein an inert gas is
passed upwardly from the bottom to replace most of the reaction gas mixture
coming from the upstream reactor. Afterwards the polymer, always maintained in
an atmosphere of the above inert gas, is transferred to a small chamber
(polymer collection zone), which is connected with the gas reaction mixture
coming from the downstream reactor. The replacement of gas reaction mixture
with an inert gas helps to reduce or to prevent polymerization in the transfer
device, thereby eliminating the deposition of polymer onto the walls of the
transfer device and blocking thereof. However, the proposed solution suffers
the disadvantage that the reaction gas mixture in the second polymerisation
reactor is considerably enriched in said inert gas. This makes it necessary to
introduce a considerable additional quantity of olefin monomers in the
downstream reactor, which may require an increase of the reactor size or an
increase of the total pressure of the gas in this reactor. Moreover, also the
transfer device described in EP-B-050013 fails in providing a continuous
transfer of polymer from the upstream reactor to the downstream reactor. This
is because the small chamber collecting the polymer must be first loaded with
the polymer, and only successively the polymer can be discharged therefrom by
opening the line connected with the gas reaction mixture from the downstream
reactor.
EP-B-503791 relates to a multistage gas-phase process for producing a bimodal
ethylene polymer blend in a sequence of two fluidised bed reactors. A high
molecular weight (HMW) polyethylene is produced in the first reactor,
successively transferred into the second reactor wherein a low molecular weight
(LMW) polyethylene is produced. The transfer device comprises a discharge tank
for collecting the HMW polymer discharged from the first reactor and a transfer
hose connected to the second gas-phase reactor. Periodically, when sufficient HMW
polymer is formed in the first reactor, the polymer and catalyst are
transferred to the discharge tank, in which the reaction gas entrained with the
polymer is vented from the top of the discharge tank. Once the desired amount
of polymer has been introduced into the discharge tank, the transfer system to
the second reactor is activated by opening a suitable valve to force the HMW
polymer into the transfer hose. The transfer hose is therefore isolated from
the upstream discharge tank and pressurized with reactor-cycle gas coming from
the downstream reactor. The transfer device described in EP 503791 is efficient
in preventing the reaction gas of upstream reactor from entering the downstream
reactor, however this transfer device cannot ensure a continuous and reliable
transfer of polymer between the two gas-phase reactors, since all the
operations of polymer discharge, degassing, pressurization, transfer and
introduction of polymer powder into the downstream reactor are performed
intermittently. Serious disadvantages are associated with the polymer transfer
systems disclosed in the above prior art transfer systems. In case a device
belonging to the transfer system is badly working or becomes clogged, the whole
polymerization plant has to be shut down. Furthermore, from the operative point
of view the discontinuous operation of polymer transfer leads to a distinct
fluctuation in the level of the polymer bed inside the downstream reactor, when
a batch of the polymer product is introduced into the downstream reactor. This
fluctuation influences some operative parameters in the downstream reactor and
may have a considerable impact on the quality of the produced polymer.
In the specific case of multistage polymerizations in an upstream slurry
reactor and a downstream gas-phase reactor the above technical problem has not
been properly faced, so that when a specific olefin monomer is wanted to be
polymerized in the upstream reactor and not in the downstream reactor, it would
be desirable to prevent said monomer from entering the downstream gas-phase
reactor.
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