Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Removing and recycling removed material from an ongoing...
Reexamination Certificate
2000-02-28
2002-09-24
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Removing and recycling removed material from an ongoing...
C526S068000, C526S070000, C526S088000, C526S901000, C422S140000
Reexamination Certificate
active
06455644
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to gas phase exothermic reactions to make particulate product in fluidized bed reactors. The invention is described with respect to olefin polymerization but is not limited to the production of polymeric products; rather, it may be practiced in connection with any exothermic reaction which is carried out in a gas fluidized bed with external cooling. It relates particularly to improvements in the condensing mode of operation in which a portion of the fluidizing gas or fluid is withdrawn from the reactor, cooled to remove the heat of reaction, partially condensed, and recycled back to the fluidized bed reactor. In the present invention, the recycle is effected by splitting the recycle stream in at least two streams directed to different areas of the reactor.
BACKGROUND OF THE INVENTION
The gas phase fluidized bed process for polymerization permits a reduction in energy requirements and capital investment compared with more conventional processes. However, a limiting factor is the rate at which heat can be removed from an exothermic reaction occurring within a fluidized bed. The heat of reaction is commonly removed from the fluidized bed by compression and cooling of a recycle stream external to the reactor vessel. The circulated recycle stream promotes fluidization of the bed within the reactor. The fluid velocity within the reactor is limited by the need to prevent excessive entrainment of solids in the fluidizing gas stream as it exits for recycle from the reactor. Hence the amount of fluid which can be circulated and cooled per unit of time to remove the exothermic heat of polymerization is limited. As polymer product is produced and removed from the fluidized bed, reactants and catalyst material are continuously supplied either to the recycle stream or directly to the reaction zone of the fluidized bed.
The quantity of polymer exothermically produced in a given volume of the fluidized bed is related to the ability to remove heat from the reaction zone. Adequate heat removal is critical to maintain a uniform temperature within the fluidized bed and also to avoid catalyst degradation and polymer agglomeration. The temperature in the reaction zone is controlled below the fusing temperature of the polymer particles. The dew point of the recycle stream is the temperature at which liquid condensate begins to form in the recycle stream. By cooling the recycle stream below the dew point temperature and then injecting the two phase mixture thus formed into the reaction zone, the heat of vaporization of liquid is available to consume a portion of the exothermic heat of polymerization. This process is known as “condensed mode” operation of a gas phase polymerization process. As disclosed by J. M. Jenkins et al. in U.S. Pat. Nos. 4,543,399 and 4,588,790 and by M. L. DeChellis, et al. in U.S. Pat. No. 5,352,749, operation in “condensed mode” permits an increase in the space time yield of the reaction system—that is, an increase in the amount of polymer produced per unit of time in a given fluidized bed reactor volume.
Below the reaction zone of the fluidized bed is a gas distributor grid plate. Its function is to provide a uniform distribution of the recycle stream into the bottom of the bed. Below the gas distributor grid plate is located a bottom head mixing chamber where the recycle stream is returned after being compressed and cooled. As disclosed by S. J. Rhee, et al., in U.S. Pat. No. 4,933149, flow deflection devices can be designed and positioned within the bottom head mixing chamber, to avoid excessive build up of entrained solids within the bottom head mixing chamber when operating without partial condensation of the recycle stream. When operating in “condensed mode”, a deflector geometry as disclosed in the '149 patent may be used to avoid excessive liquid flooding or frothing in the bottom head mixing chamber. However, as the condensing level is increased to further enhance heat removal and space time yield, excessive amounts of liquid can exist in the bottom head mixing chamber. This can lead to liquid pooling and instability problems.
As disclosed by Jenkins, et al., in U.S. Pat. No. 4,543,399 and by Aronson in U.S. Pat. No. 4,621,952 the polymer product is intermittently withdrawn from the fluidized bed at an elevation above the gas distributor grid plate. At increasing levels of partial condensation of the recycle stream the likelihood increases that undesirably high levels of liquid phase may exist in lower portions of the fluidized bed. Unfortunately during a product discharge event liquid can be carried out of the reactor along with the granular polymer and gases. As the liquid expands and vaporizes within the discharge equipment, temperature reduction and pressure elevation can occur. This can reduce the fill efficiency of the discharge system. This reduction in fill efficiency reduces the production capacity of the facility and increases the raw material usage of the process. Accordingly, it has been difficult to increase the liquid content in the recycle stream to enhance the efficiency of removing the heat of reaction.
Chinh et al, in U.S. Pat. No. 5,804,677 describe the separation of liquid from a recycle stream; the separated, collected liquid is injected into the fluidized bed above the gas distributor plate. The present invention also injects recycle liquid above the distribution plate, but applicants' liquid is handled as a liquid/gas mixture and as a more or less predetermined fraction of the recycle stream, as a slip stream, divided simply and directly in the recycle conduit. Because of the applicants'
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manner of separating, we are able to enhance the ratio of liquid to gas in the slip stream as compared to the withdrawn recycle stream, and thus simply and directly, without additional or special equipment, improve heat exchange efficiency and enhance the space/time yield of the process.
SUMMARY OF THE INVENTION
Our invention comprises splitting the recycle stream, after compression and cooling, into at least two streams. One of the streams is returned to the distributor grid plate or similar device below or near the bottom of the bed and the other(s) are returned to the fluidized bed at one or more points above the distribution grid plate. The stream is split by a conduit segment designed for the purpose, sometimes herein called a splitter.
Preferably, the recycle stream is divided into two streams, the smaller of which is 5 to 30 percent of the total recycle stream and contains an enrichment of the liquid portion as a function of the relative momentums of the liquid and gas components of the recycle stream, impacting in the splitter, the liquid droplet size, and the particular configuration of the splitter. The liquid content (percentage by weight) of the smaller stream is preferably enriched to a percentage 1.01 to 3.0 times, more preferably 1.1 to 2.5 times that in the stream prior to separation. The larger of the separated streams, having a lower liquid concentration but a higher volume, is recycled to the bottom head mixing chamber of the reactor vessel and introduced into the reaction zone in a uniform fashion more or less conventionally through a gas distributor grid plate. The smaller stream or streams having an enriched liquid phase, is (or are) recycled into the reaction zone at an elevation above the gas distributor plate. Because of the lower ratio of liquid to gas in the larger stream as compared to the original cooled/condensed stream, only a minimal disturbance of the fluidized bed is imparted. We are thus able to inject higher quantities of recycled liquid into the bed without causing difficulties in the product withdrawal system.
An attractive novel feature of our modified recycle technique is that the separation of the recycle stream may be conducted without using mechanical equipment such as separators, hydrocyclones, demisters, scrubbers, entrainment collection devices, pumps, compressors or atomizers. Rather, by withdrawing the small two phase stream or streams fro
Howley Timothy Joseph
Olson Robert Darrell
Teskin Fred
Union Carbide Chemicals & Plastics Technology Corporation
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