Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Removing and recycling removed material from an ongoing...
Reexamination Certificate
2001-04-04
2002-10-29
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...
C526S061000, C526S068000, C526S348200, C526S348500, C526S348600
Reexamination Certificate
active
06472482
ABSTRACT:
The present invention relates to a gas-phase polymerization apparatus and a gas-phase polymerization process for preparing polyolefins, preferably polyethylene, in which the fluid monomeric component, preferably ethylene, and, optionally, one or more comonomers is/are polymerized in the polymerization zone of at least one fluidized-bed reactor in the presence of one or more catalysts under reaction conditions where the fluidizing gas is circulated to remove the heat of polymerization or reaction and the polymerization product is taken from the circuit and the corresponding amount of monomer and, optionally, comonomers are fed in.
Gas-phase polymerization processes are economical processes for the polymerization of olefins such as ethylene and propylene or for the copolymerization of olefins with C
3
-C
8
-olefins. Such gas-phase polymerization processes can be configured, for example, either as gas-phase fluidized-bed processes or as stirred gas-phase processes. Such processes are described, for example, in EP-A-0 475 603, EP-A-0 089 691 and EP-A-0 571 826.
A characteristic of the gas-phase fluidized-bed process is that the bed consisting of polymerizing polymer particles is kept suspended by a gas mixture flowing in from below. The heat of polymerization or reaction liberated is carried from the reactor by the abovementioned gas mixture. The fluidizing gas is cooled by indirect cooling in a heat exchange zone located outside the reactor and is generally returned to the reactor through a gas distributor plate (circulating gas or fluidizing gas).
Gas-phase fluidized-bed processes for preparing polyolefins are customarily carried out in a vertical, cylindrical reactor at superatmospheric pressure and elevated temperature. For example, the gas-phase fluidized-bed polymerization of ethylene is carried out at an internal reactor pressure of about 25 bar and a temperature of about 110° C.
The monomer fed in with the fluidizing gas is reacted by means of a catalyst and, optionally, cocatalysts and various auxiliaries in the fluidized-bed reactor to give the corresponding polyolefins, usually in the form of a pulverulent polymerization product. The particles of polymer powder which essentially form the fluidized bed are generally discharged from the fluidized-bed reactor via a continuously open or intermittently open orifice. The (substantial) separation of gas and powder is carried out in one or more precipitation systems at lower pressure. In addition, the fluidized-bed reactor generally has a disengagement zone at its upper end and additionally, if desired, a plurality of downstream stages to avoid or to reduce entrainment of particles.
The polymerization of olefins carried out in the gas-phase fluidized-bed process liberates a considerable quantity of heat of polymerization or reaction which is essentially taken up by the fluidizing gas. As a result, the fluidizing gas leaving the fluidized-bed reactor at its upper end has a correspondingly higher temperature than when it enters the fluidized-bed reactor. The circulated fluidizing gas therefore has to be cooled in a heat exchange zone to remove the heat of reaction or polymerization prior to being returned to the reactor.
A number of apparatuses and processes for cooling the circulated fluidizing gas are described in the prior art. In all the previously disclosed processes and apparatuses, cooling is carried out by means of indirect heat exchange, for example over conventional heat exchangers such as shell-and-tube heat exchangers. The corresponding cooling apparatuses for indirect heat exchange will hereinafter be referred to as recycle gas coolers.
Part of the heat of reaction or polymerization evolved is generally removed via the wall of the usually uninsulated reactor. JP 070/620/09-A and JP 09/497/03-A describe apparatuses for the indirect cooling of the reactor wall, for example by means of pipes through which cooling water flows.
For a long time, the recycle gas was cooled by means of indirect cooling to a recycle gas temperature slightly above the dew point of the recycle gas. The dew point is the temperature at which the (recycle) gas begins to condense. One of the reasons why the recycle gas was always cooled only to a temperature just above the dew point was the assumption that the introduction of liquid into a gas-phase fluidized-bed reactor would unavoidably lead to formation of lumps and/or to blockage of the distributor plate.
More recently, a significant increase in the performance (space-time yield) of the gas-phase polymerization process has been able to be achieved by deliberately cooling the heated recycle gas to below the dew point. When the resulting two-phase mixture consisting of condensed and gaseous fluid is introduced, the condensate vaporizes and thus takes up part of the heat of reaction (known as “Condensed Mode”). EP 0 089 691-B1, U.S. Pat. No. 4,543,399, U.S. Pat. No. 4,588,750 and U.S. Pat. No. 5,352,749 claim indirect cooling where at least part of the recycle gas is cooled below the dew point to form a two-phase mixture and this is recirculated to the reactor. Increasing the proportion of condensate by deliberate addition of higher-boiling components which thus condense at higher temperatures is also claimed. Furthermore, U.S. Pat. No. 4,588,750 describes the introduction of monomers in liquid form into the reactor below the fluidized bed. The introduction of the abovementioned components which vaporize under the reaction conditions effects additional evaporative cooling in the reactor.
U.S. Pat. No. 5,352,749 likewise discloses indirect cooling in which, however, the liquid condensate phase is separated from the two-phase mixture of the recycle gas before being returned to the reactor. An analogous procedure is employed in BP's “Innovene” polyethylene process (WO
9428032).
In all the abovementioned processes, the heat of reaction evolved in the gas-phase polymerization is removed by means of an indirect cooling apparatus. The process step of indirect cooling or partial condensation of the fluidizing gas, which is at the same time the circulated gas, is generally carried out in one or more heat exchangers. Typically, shell-and-tube heat exchangers or plate heat exchangers are used. A fundamental disadvantage of indirect cooling is that gas-side heat transfer is relatively poor. Considerable specific heat transfer areas are therefore necessary, which results in high equipment costs. In addition, the poor heat transfer intrinsic in the mode of construction also has an unfavorable effect on the energy consumption. Furthermore, the control of indirect cooling apparatuses is fundamentally very poor or is possible only with substantial time delays, so that they generally do not allow fine regulation of heat removal. Although the specific heat transfer area can be reduced when cooling to temperatures below the dew point because of the resulting two-phase system, very large heat transfer areas are still necessary, as indirect cooling processes are generally relatively ineffective.
In addition, a certain amount of finely divided polymer is carried from the reactor by the circulating gas and introduced into the circulated gas system. These polymer particles contain active catalyst and can thus also polymerize further in the circulated gas system. If these particles deposit in the circulated gas system, fouling can result in these places. These deposits can cause malfunctions (e.g. blockage of the cooler) and can also partly flake off again. The flaked-off deposits can then quickly block holes in the gas distributor plate of the reactor and thus necessitate shut-down and costly cleaning of the reactor. If such pieces of the deposits get through the gas distributor plate into the reactor, the product quality is adversely affected by these particles since they result in formation of specks. Particularly in the case of products for film applications, material which does not conform to specifications may be produced as a result.
It is an object of the present invention to provide a gas-phase polymerizat
Evertz Kaspar
Sandkühler Manfred
Schicketanz Walter
Basell Polyolefine GmbH
Cheung William K
Keil & Weinkauf
Wu David W.
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