Dry product discharge from a gas phase polymerization...

Details Dry product discharge from a gas phase polymerization... Dry product discharge from a gas phase polymerization...

2000-02-28

2002-10-29

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S065000, C526S070000

Reexamination Certificate

active

06472483

ABSTRACT:

TECHNICAL FIELD
This invention relates to the continuous or intermittent removal of particulate polymer product from a fluidized bed system operating in the condensing mode, that is, where liquid is added, recycled and condensed outside the reactor to enhance the removal of the heat of reaction.
BACKGROUND OF THE INVENTION
The widely used “Unipol” fluidized bed olefin polymerization process employs two tanks in series for the intermittent removal of granular polymer from the fluidized bed, generally as is illustrated in Aronson's U.S. Pat. No. 4,621,952. In the original design for use with a dry gas phase reaction system, the discharge nozzle was located near the bottom of the fluidized bed. Recent advancements in heat removal and reactor static control have led to partial condensation of the reactor feed gas. The two-phase vapor and liquid mixture enters the reactor fluidized bed from the bottom. Thus significant quantities of liquid may exist at and near the bottom of the fluidized bed. The inlet fluid is ultimately completely vaporized, absorbing the heat of reaction as it moves upward through the polymerization zones of the fluidized bed reactor. But unfortunately during a discharge event, liquid may be carried out of the reactor along with the granular polymer when it is removed for further processing and shipment. Significant cooling can occur within the product discharge tanks as the liquid vaporizes.
Vaporization of liquid in the product discharge system may cause the pressure in the blow tank and other parts of the system to approach dangerous and/or maximum allowed working pressures, as the original design for the equipment for a dry system did not anticipate such vaporization. Particularly in the case of polypropylene production, this can be a limiting production rate factor.
The pressure in the product discharge tanks is affected particularly by flashing of the liquid from the product as soon as it enters the relatively low pressure product discharge tank. In addition, any liquid which remains in the liquid phase takes up volume in the tank which could be occupied by resin product, and tends to reduce flowability of the resin. The upward flow of gas vaporizing from liquid originally settling in the bottom of the tank provides resistance to the downward flow of the granular product, thus also retarding the introduction of the product to the tank.
Low temperatures may occur in the product discharge tank or the blow tank as the liquid evaporates, especially when the condensing agent is propylene or propane.
The liquid that escapes with the resin out of the reactor must be recovered into the reaction system to achieve economical use of feedstocks. Commercial systems are designed with elaborate monomer and feedstock recovery schemes for this purpose. One of these is the Unipol improved product discharge system, sometimes called the IPDS. Regardless of the recovery scheme, it is clearly advantageous to reduce the amount of liquid that escapes with the resin. If the amount of liquid reaching the product discharge tank is minimized, the size and costs of the recovery equipment can be reduced and less feedstock may be lost.
The liquid's flash may also reduce the resin's temperature in the product discharge system. This is undesirable where the solubility of monomer and/or condensing agent in the product is enhanced by lower temperatures.
Liquid may also contribute to progressive increases in the baseline pressure of a product discharge system over multiple resin discharge cycles. Baseline pressure is that pressure in the product discharge tank when the discharge valve first opens or the pressure in an empty tank as the system cross-ties. An increase in baseline pressure reduces monomer savings effected by a cross-tie to another tank.
Generally, the prior art has not dealt with the above problems caused by the ever-increasing quantities of liquid present in the system and liable to be removed from the reactor with product. Patents disclosing product discharge configurations after the above mentioned Aronson patent 4,621,952 have not addressed minimizing the amount of liquid in the product. See, for example, DeLorenzo U.S. Pat. No. 4,535,134, which employs weirs in a horizontal reactor. The weirs tend to assure that the product removed has already settled at levels higher than half the height of the reactor, but in the end the product drain used at the end of the reactor is at its bottom. The apparatus of EP 0 830 892 A1 removes product from the top of the fluidized bed primarily by gravity in order to minimize the amount of gas removed with the product.
The sloped discharge (of EP 0 890 892) is said to be from 0.6 to 0.95H. This allows sufficient height that the discharge system tanks and associated hardware can be built at ground level when the reactor vessel is essentially located at ground level. The 0.6H to 0.95H specification also accommodates the height required in the construction of the sloped chute from the reactor port to the resin receiving vessel. The removal of product high in the reactor also aids in the removal of resin fines that are prone to accumulate near or at the top of the fluidized bed. That is particularly a problem when operating with a cyclone separator in the gas recirculation line from the top of the reactor to the bottom. Fines in the recirculating gas are separated and returned to the top of the fluid bed by the cyclone. See Bontemps et al U.S. Pat. No. 5,382,638.
As indicated in the above mentioned Aronson patent 4,621,952, product may be removed from the reactor to the product discharge tank by utilizing the difference in pressure between the upper and lower ports of the fluidized bed. While there is an initial rush of polymer particles into the discharge vessel when the discharge valve is opened due to gravity and the pressure difference between the reactor and the discharge vessel, this is ineffective in filling the vessel to near its capacity within a practical time limit. To increase the rate of product discharge after the initial rush, a vent is opened to the upper regions of the reactor to take advantage of the pressure difference, typically from 2 to 12 psi, between the higher pressure low regions and the low pressure upper regions of the bed. Some of the upward flowing fluidizing gas is caused to leave the bed and pass through the discharge vessel to reach the upper region of the bed, and in so doing, product is conveyed from the reactor into the discharge vessel. This greatly increases the amount of product removed during each discharge cycle.
While we do not purport to coin any new terms, it may be useful to discuss the meanings of a few terms used in this application, as there may be some disagreement among practitioners of the art as to their meanings. For example, we have construed the letter “H” as used in EP 0 830 892 A1 to mean the height of the reactor wall around the fluidized bed as illustrated in that patent—in other words the height of the fluidized bed as defined by the straight or cylindrical wall only, beginning with the distributor plate, terminating at the top of the straight or cylindrical wall, and not including the expanded zone above the straight or cylindrical wall or any conical section such as is commonly used to transition from the straight (cylindrical) section to the expanded section. This is the meaning used in the present application. When we state that a port in the reactor wall is located at a certain distance from the distributor plate, such as 0.15H, we mean that the center of the port is at that vertical distance from the distributor plate. Also, the term “condensing mode” is used to include a fluidized bed process in which fluid from the reactor is removed, cooled and condensed to remove the heat of reaction, and returned to the reactor. The fluid may contain from 1% to 95% liquid by weight after cooling and condensing, and may or may not contain non-reactive materials added to enhance the efficiency of heat removal. Thus, as an example, where 20% by weight of the recycled fluid enter

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