High condensing mode polyolefin production under turbulent...

Details High condensing mode polyolefin production under turbulent... High condensing mode polyolefin production under turbulent...

1999-10-21

2002-05-21

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...

C526S059000, C526S061000, C526S068000, C526S072000, C526S351000, C526S352000, C526S348000

Reexamination Certificate

active

06391985

ABSTRACT:

TECHNICAL FIELD
This invention relates to the production of polymers in fluidized beds, particularly in fluidized bed processes for the polymerization of olefins, adjusted to operate turbulently to facilitate high levels of liquid in the recycled fluid.
BACKGROUND OF THE INVENTION
The production of polyolefins in fluidized beds requires that the heat of reaction be removed in order to maintain appropriate temperatures for the desired reaction rate. In addition, the temperature of the vessel cannot be permitted to increase to the point where the product particles become sticky and adhere to each other. The heat of reaction is commonly removed by circulating the gas from the fluidized bed to a heat exchanger outside the reactor and passing it back to the reactor.
The earliest such recycle systems were based on the assumption that it would be inefficient, or inoperable, to cool the recirculating gas below its dew point so that liquid would be introduced into the reactor through the recycle process. However, operation in the “condensing mode” has become quite common in the art—see Jenkins U.S. Pat. Nos. 4,543,399 and 4,588,790. In accordance with the teachings of these patents, an inert liquid may be introduced into the recycle stream to increase its dew point. The resulting ability to remove greater quantities of heat energy in less time has increased the production capacity of the typical exothermic fluidized bed reactor.
More recently, in U.S. Pat. Nos. 5,352,749, 5,405,922, and 5,436,304 (see column 12, lines 4-17), higher levels of liquid have been shown to be practical. Griffin et al, in U.S. Pat. No. 5,462,999, observe a range of bulk density functions Z, which include dependence on temperature, pressure, particle characteristics and gas characteristics. As in the Griffin et al '999 patent, which is incorporated by reference, we refer herein to the bulk density function Z, defined (col. 12, lines 38-47 of Griffin '999 and col. 12, lines 31-42 of Griffin et al U.S. Pat. No. 5,436,304) as
Z
=
(
p
bf
-
p
g
)
/
p
bs
(
p
s
-
p
g
)
/
p
s
where p
bf
is the fluidized bulk density, p
bs
is the settled bulk density, p
g
is the gas density and p
s
is the solid (resin) density. The bulk density function Z can be calculated from process and product measurements.
Fluidized bulk density (FBD), and particularly the ratio of fluidized bulk density to settled bulk density (SBD), are asserted to be limiting factors for stable operation where higher quantities of liquid are used in the recycle stream. DeChellis and Griffin, in U.S. Pat. No. 5,352,749 (also incorporated in its entirety by reference) place an upper limit of 5.0 feet per second (1.5 m/sec) on the superficial gas velocity (“SGV”) within the reactor—see column 8, lines 31-33. The various perceived limits on operating conditions have inhibited workers in the art from increasing the level of liquid in the recycle stream and from venturing into the realm of turbulence in the fluidized bed. DeChellis and Griffin, in U.S. Pat. No. 5,352,749, maintain the ratio of FBD/SBD above 0.59 (col. 4, line 68), stating “as a general rule a reduction in the ratio of FBD to SBD to less than 0.59 may involve risk of fluidized bed disruption and is to be avoided.” (Col. 5, lines 10-12).
Govoni et al, in U.S. Pat. No. 5,698,642 (col. 2, line 40), refer to the “turbulence” generated by the grid (distributor plate) which distributes the liquid into the bed of polymer in the DeChelllis et al '749 patent, but this is not turbulence as defined (see below) in turbulent fluidization. Unlike the present invention, Govoni et al operate under fast fluidization conditions.
Definition of the Turbulent Regime
There are at least five different fluidization regimes. In order of increasing gas velocity (U) or decreasing solids concentration, they are particulate fluidization (for group A particles only), bubbling fluidization, turbulent fluidization, fast fluidization, and pneumatic transport. Gupta and Berruti also describe “dense phase conveying,” a fluidization regime that qualitatively can be considered an extension of the turbulent regime where there is no dilute freeboard above the bed as is common in olefin polymerization, resulting in high solids carryover at the top of the fluid bed reactor. Gupta and Burruti, Fluidization IX, 1998, p. 205. We include dense phase conveying in the definition of turbulent fluidization for purposes of our invention.
A turbulent regime is not simply a regular dense bed of bubbling fluidization regime having substantial freeboard activities. The turbulent regime has distinct features differing from those of the bubbling and fast fluidization regimes. Most available models and correlations developed for bubbling fluidization regimes or fast fluidization regimes cannot be applied for turbulent fluidization regimes.
The mean amplitude of pressure fluctuations in the fluidized bed has been observed as having a noticeable downturn as the superficial gas velocity increased to a certain point. The peak mean amplitude fluctuation was taken as the velocity for the beginning of a transition to turbulent fluidization, and denoted U
c
. See Lee, G. S. and Kim, S. D., Journal Chemical Engineering (Japan) vol. 21, No. 5 (1988), 515. U
c
is defined as the velocity at which amplitude of pressure fluctuations peak. We note that it marks the transition from the bubbling regime to the turbulent regime, and accordingly we sometimes call it herein the transition velocity. In addition to the amplitude of pressure fluctuations, characteristic indicia of pressure fluctuation intervals, standard deviation of pressure fluctuation, skewness and flatness of pressure fluctuations, and power spectral density function of pressure fluctuations may also be observed at U
c
according to Lee and Kim. However, their correlation of the Archimedes Number to the critical Reynolds Number for turbulence is not applicable to pressurized fluid bed polymerization. The velocity at which the mean amplitude of pressure fluctuations level off as the gas velocity is increased beyond U
c
is defined as U
k
, as will be illustrated herein in FIG.
3
. We take the appearance of U
k
as marking the termination of turbulent fluidization and the onset of fast fluidization, as the superficial gas velocity increases.
The structure of a fluidized bed changes when the gas velocity exceeds U
c
. The most important difference is in the bubble behavior. Specifically, the bubble interaction is dominated by bubble coalescence at gas velocities smaller than U
c
, while it is dominated by bubble break-up at gas velocities greater than U
c
(e.g., Cai et al., “
Effect of Operating Temperature and Pressure on the Transition from Bubbling to Turbulent Fluidization
”, AIChE Symposium Series—Fluidization and Fluid Particle Systems—Fundamentals and Application, No. 270, v. 85 page 37, 1989;
Characterization of the Flow Transition between Bubbling and Turbulent Fluidization
” by Ahmed Chehbouni, Jamal Chaouki, Crisstopher Guy, and Danilo Klvana, Ind. Eng. Chem. Res. 1994, 33, 1889-1896. The bubble/void size in the turbulent regime tends to decrease with the increase of gas velocity due to the predominance of bubble break-up over bubble coalescence. This trend is opposite to that in the bubbling regime. Thus, with sufficiently high gas velocity, bubble/void size can be reduced to an order of magnitude similar to the particle size. This high gas velocity, called the transition velocity, demarcates the diminishing of bubbles in the turbulent regime and a gradual transition to lean-phase bubble-free fluidization. As a result of the dominant break-up tendency of bubbles/voids, more small bubbles/voids with relatively low rise velocities and longer residence time exist in turbulent systems, which leads to a more significant dense bed expansion than that in the bubbling regime, and therefore a lower fluid bed density. Bubbles/voids in the turbulent regime are less regular in shape compared with those in bubbling beds. At relatively high gas velocities in the turbulent regime, the clear

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