Patent Application
20060251547
1. Field of the Invention
The present invention relates to an apparatus for the production of polycondensation products, such as linear polyesters and co-polyesters. More particularly, the present invention relates to improved tray designs for use in vertical oriented polymerization reactors.
2. Background Art
Processes for producing polymeric materials such as polyesters and co-polyesters via polycondensation reactions involve the liberation of by-products as the polymeric functional groups of the molecules react with one another to produce longer molecular chain molecules. Typically, the extraction of these liberated byproduct molecules from the reaction mixture is necessary in order to drive the molecular build-up of the polymer. If the by-product compounds were not removed, chemical equilibrium will inhibit the length of the formed polymeric chain. In many of these polycondensation reaction systems the preferred method for extracting the liberated by-product is to vaporize the by-product out of the reaction mixture.
Various reactor designs and multi-step reaction systems have been designed and operated to facilitate the vaporization of by-products and the associated production of polycondensation materials. The most economical design for such polycondensation reactions (at least for the production of low to moderate molecular weight polymeric materials) is a series of stirred tank reactors. In these reactor systems large quantities of materials can be produced through the use of mechanical agitation, thermosiphon reboilers, and/or simple bubble agitation to enhance heat transfer and liquid-vapor surface area renewal. Unfortunately, the viscosity of the polymeric melts increase dramatically as the degree of polymerization (“DP”) increases. Accordingly, because of the practical limitations of agitator designs, the high viscosity of these materials greatly decreases the capability of renewing the liquid-vapor surfaces and hence decreases the mass transfer efficiency of the stirred tank reactor.
In addition to the short comings set forth above, other operating parameters may also be restricted in the polycondensation process. For example, higher temperatures may be desirable to increase reaction kinetics and volatility of reaction by-products. Higher volatility of the by-products decreases by-product concentration in the reaction mixture, thereby furthering the polymerization reaction. However, the temperature sensitivity of the polymeric material to degradation reactions limits the use of increasingly higher temperature as a means of furthering the degree of polymerization. Similarly, the volatility of the by-products may be further increased by the use of low operating pressures. However, use of extremely low operating pressures is limited by the cost of achieving low operating pressures and the amount of reactor vapor space needed to prevent entrainment of polymer into the vacuum source. Moreover, the depth of the polymeric pool can inhibit the effective use of the reaction volume in low-pressure polycondensation reactors. Specifically, excessive depth of the reaction mixture increases the diffusion and convection paths that volatile byproducts must travel before escaping. Furthermore, as the depth of the polymeric pool increases, the deeper portions of the pool are subjected to greater hydrostatic pressure. Higher local pressures within the liquid inhibit the formation of by-product bubbles, which hinders the liberation of the by-products and hence the effective use of the reaction volume for furthering polymerization.
For the reasons set forth above, increasing the degree of polymerization requires replacement of simple stirred tank reactors with specialized reaction equipment. Such specialized equipment must overcome one or more of the operating limitations noted above to achieve the desired degree of polymerization. Currently, there are two fundamental approaches for enhanced liquid-vapor surface renewal that are best described as the dynamic approach and the static approach.
The first approach might be termed the dynamic approach in that it involves the use of moving mechanical devices to enhance liquid-vapor surface renewal. As noted above, enhanced liquid-vapor surface renewal facilitates the liberation of the by-products. With the dynamic approach, seals are needed around the rotating shaft or shafts that pass through the reactor walls. These seals must be maintained in order to prevent air from leaking into the reactor. Also with the dynamic approach, as the size of the vessel and the viscosity of the product increase, the size of the mechanical components must increase in order to handle the increase in load. The second approach can be referred to as the static approach in that no moving devices are used for the liquid-vapor surface renewal. This later approach uses gravity in combination with vertical drop to create thin polymeric films. Typically, such polymeric films flow between trays during the vertical drop. The thin polymeric films combined with shearing and surface turnover effects created by vertical falling films drive the polymerization reaction by enhancing the liberation of by-products.
Prior art patents which disclose the use of gravity in combination with vertical drop include: U.S. Pat. No. 5,464,590 (the '590 patent), U.S. Pat. No. 5,466,419 (the '419 patent), U.S. Pat. No. 4,196,168 (the '168 patent), U.S. Pat. No. 3,841,836 (the '836 patent), U.S. Pat. No. 3,250,747 (the '747 patent), and U.S. Pat. No. 2,645,607 (the '607 patent). Early tray designs used vertically spaced circular trays (full circle in combination with hollow circle, and segmented circular) that utilized most of the cross-sectional area of the vessel. These circular tray reactors use a large portion of the available pressure vessel's horizontal cross-section for liquid hold-up. In some designs, a circular tray was followed by a hollow circle tray thus forming a disc-and-doughnut arrangement. Thus polymer flowed over a circular edge as it passed from tray to tray. The liberated gas by-product thus flowed through circular and annular openings. In other designs, the trays were segmented to provide a straight edge for the polymer to flow over before dropping to the next tray. The segmented tray design also provided open area between the straight edge over which polymer flowed and the vessel wall though which the gas by-product could pass. With both designs however, the vaporized by-products from the trays was forced to flow through the same space as the polymer melt flow. To address this concern, the diameter of the circular trays was made somewhat less than the reactor vessel's diameter. The resulting annular space was used to allow vapor traffic to escape each tray and travel to the reactor vessel's vapor discharge nozzle along a path external to the path of the polymer flow. A shortcoming of the simple circular tray designs is the existence of dead zones (very slow moving or stagnant regions on the trays). The polymer in these stagnant regions tends to overcook, become excessively viscous, cross-link and/or degrade, and as a result slowly solidify. The net result is a loss of effective reaction volume.
The next generation of designers changed the shape of the trays from circular to other geometric shapes. They eliminated dead zones, which are not entirely effective as reaction volume. The elimination of dead zones also improved, product quality since the dead zones are regions that produce high levels of degradation by-products and poor color due to the overcooking of the polymer. Unfortunately, these noncircular shaped trays did not increase the effective use of the cylindrical pressure vessel's cross-sectional area.
The basis for more recent inventions of the '590 patent and the '419 patent is a hollow circular tray which more efficiently utilizes the cross-sectional area of a cylindrical pressure vessel while providing polymer melt flow paths which minimize liquid dead zone regions and prevent channeling. The net result was an approximate 40% increase in tray area available for liquid retention as compared to the non-circular shaped trays. The central opening in the trays provided a chimney through which the vapor by-products are removed.
However, as set forth above, the depth of the polymeric pools can also inhibit the effective use of the reaction volume at low operating pressures. At a given operating pressure (vacuum level), the impact of polymer depth increases in proportion to the degree of polymerization.. This is due to reduction of the chemical equilibrium driving force for polymerization as the concentration of polymer end groups are reduced through the growth of the polymer chains. Hence, to get acceptable results, the mechanisms for liberating polycondensation by-products from the polymer melt must be further enhanced. At higher degrees of polymerization this is necessary so that sufficiently low levels of by-products remain in the melt enabling the polymerization to proceed efficiently. However, another important factor is that the viscosity increases substantially as polymerization proceeds to higher degrees of polymerization.
At a sufficiently high viscosity, horizontal trays cannot achieve the desired combination of both high polymer throughput and shallow polymer depths. The designs of Lewis et al. (the 168 patent) achieve a degree of control over the polymer depth by having the polymer flow down sloping trays. The slopes of the successive trays are increased to account for the expected increasing viscosity of the polymer as it polymerizes along its course. In the present invention described herein, substantially vertical surfaces are desirable for polymer systems with higher throughputs, and even higher viscosities, because of the reduced film thickness through which the liberated gas must pass.
The design of the '168 patent (roof-and-trough trays) also achieved some degree of control over polymer depth by splitting the polymer melt into two equal streams (with one flow path being a mirror image of the other flow path) that traverse from the top to the bottom of the reactor over sloped trays. Lewis' design innovation over simple sloped trays was a reduction of the reactor vessel volume needed to enclose the trays within a vacuum environment. By splitting the polymer flow the vertical dimension (vertical drop) needed for a tray to achieve a desired slope and hence a desired polymer depth was reduced. The roof-and-trough configuration cuts the horizontal length of the tray that each half of the polymer flow must traverse before dropping to the next tray. Since each half of the polymer flow traverses half the horizontal distance, the residence time for each is approximately the same as a simple sloped tray while using less total vertical height.
As the production rates are increased, the roof-and-trough design concept can be extended by splitting the polymer streams into more equal streams, generally in binary fashion—two, four, eight . . . . Thus, good utilization of the reactor vessel volume is maintained as the vessel is increased in size to accommodate the polymer throughput.
However, even with the roof-and-trough tray design of Lewis, utilization of the reactor vessel volume decreases as the desired degree of polymerization is pushed higher and/or the mass transfer versus residence time operating window is narrowed to achieve better quality. As the targeted degree of polymerization is pushed higher, the polymer viscosity increases, thus to maintain the same polymer depth requirements steeper tray slopes are required. Similarly, if mass transfer is to be increased by targeting shallow polymer depths, then steeper trays are needed. At some point the slopes become essentially vertical (greater than 60° slope from the horizontal), and appreciably thinner depths for a given combination of throughput and viscosity cannot be achieved by further changing the slope. In this region of high throughputs, targeted shallow depths, and high viscosity, the film generation and film support structures of the present invention described herein increase the number of polymer sheets within a given reactor vessel cross-sectional area, thereby achieving high throughputs and better mass transfer.
Accordingly, there is a need for improved designs for film generation and film support in polycondensation reactors that makes more efficient utilization of space in a vertical, gravity flow driven polymerization reactor for combinations of high viscosity, high throughput, and thin films.
The present invention overcomes one or more problems of the prior art by providing in one embodiment a bundle assembly of static internal components for a vertical, gravity flow driven polymerization reactor for combinations of high viscosity, high throughput, and thin polymer melt films. The present invention is an enhancement of earlier designs that also used the approach of gravity and vertical drop to achieve the desired degree of polymerization. Such earlier designs are disclosed in U.S. Pat. No. 5,464,590 (the '590 patent), U.S. Pat. No. 5,466,419 (the '419 patent), U.S. Pat. No. 4,196,168 (the '168 patent), U.S. Pat. No. 3,841,836 (the '836 patent), U.S. Pat. No. 3,250,747 (the '747 patent), and U.S. Pat. No. 2,645,607 (the '607 patent). The entire disclosures of these patents are hereby incorporated by reference. The present invention provides large surface areas over which the liquid is in contact with the atmosphere of the reactor while still attaining sufficient liquid holdup times for the polymerization to take place, by means of the novel components within what will be termed ‘the bundle assembly’. The reactor vessel provides a means for controlling both the pressure and temperature in the space surrounding the bundle assembly.
The bundle assembly of the invention includes one or more stationary film generators. The bundle assembly further includes one or more stationary arrays of film support structures, wherein arrays are separated by film generators. Typically, each array of film support structures is arranged in one or more rows characterized by all of the film support structures within a row being at the same elevation (i.e., height). According to the vertical arrangement of the components in the bundle assembly within a reactor vessel, the polymeric melt cascades down the vertical length of the vessel interior.
The film generator is any device that subdivides a flowing polymer stream into two or more independently flowing streams with a resultant increase in the number of free surfaces. By dividing the polymer melt, it can be more uniformly applied to the film support structures located below it. Furthermore, the film generators create large amounts of free surface area for the flowing polymer streams, which are retained and\or extended by the film support structures.
The array of film support structures provides solid surfaces upon which the polymer streams from the film generator flow. Each of the film support structures has a first side and a second side. A portion of each subdivided polymer stream flows over the first side, and a second portion of the subdivided polymer stream flows over the second side. In this manner, the film support structure is coated with flowing polymer. The film support structures are usually oriented at least 60 degrees, and preferably about 90 degrees, from the horizontal plane. A row of film support structures can be created in a number of ways. For example, a row can be formed by mounting at an equal elevation a plurality of horizontally spaced planar film support structures. For such an array, the linear or normal spacing between the planes of adjacent film support structures is preferably constant for a given row. Alternatively, a row can be formed by arranging the film support structures about a substantially vertical line. For this latter case, the angular spacing between adjacent film support structures is preferably constant within a given row. The film support structures are not required to be planar. For example, an array of film support structures can be created from a series of concentric cylinders or ellipses. In another variation, an array can be created by spiraling the film support structure about a vertical line.
Optionally, multiple film generators and arrays of film support structures are vertically arranged to form the bundle assembly. The vertically arranged rows of film support structures typically have a highest positioned row, a lowest positioned row, and optionally one or more intermediately positioned rows. In turn, each row includes one or more film support structures that are positioned such that when the polymeric melt contacts a film support structure the polymeric melt moves in a downward direction under the force of gravity. The arrangement of the rows is such that each row (except the lowest row) transfers the polymeric melt to a lower vertically adjacent film generator or row of film support structures. The presence of a film generator between rows of film support structures facilitates changing the number, orientation, or shape of the surfaces of the film support structures from one row to the subsequent lower row.
FIGURE SA is a perspective view of a row of film support structures utilizing planar surfaces with equal angular spacing;
FIGURE SB is a perspective view of a film generator positioned over the film support structures of
Reference will now be made in detail to the presently preferred compositions or embodiments and methods of the invention, which constitute the best modes of practicing the invention presently known to the inventors.
In an embodiment of the present invention, a bundle assembly adapted to be placed in a reactor for polymerizing a polymeric melt is provided. With reference to
Typically, each of film support structures 38, 40, 42, 44 are substantially vertical with an angle equal to or greater than about 60 degrees between each film support structure and a horizontal plane. In a variation of the invention, each film support structure of the plurality of film support structures 38, 40, 42, 44 is substantially vertical with an angle equal to or greater than about 80 degrees between each film support structure and a horizontal plane. In another variation of the invention, each film support structure of the plurality of film support structures 38, 40, 42, 44 is substantially vertical with an angle from about 80 to preferably about 90 degrees between each film support structure and a horizontal plane. In yet another variation of the invention, each film support structure of the plurality of film support structures 38, 40, 42, 44 is substantially vertical with an angle of about 90 degrees between each film support structure and a horizontal plane. Each film support structure of the plurality of film support structures 38-44 are positioned such that when polymeric melt 46 contacts a film support structure of the plurality of film support structures 38-44, polymeric melt 46 moves in a downward direction under the force of gravity. Moreover, when additional arrays 26, 28, 30 are present, each array of vertically arranged arrays 24, 26, 28 transfer polymeric melt 46 to a lower vertically adjacent array.
The parallel arrangement of the surfaces of the film support structures in
With reference to
The film generator is any device that can be used to uniformly subdivide the polymer flow onto the film support structures. Arrays of rods, bars, pipes, half-pipes and angles can be easily arranged to form film generators for planar film support structures that are parallel. For more complex film support structures, a film generator can be formed from a plate by adding arrays of appropriately positioned openings. With reference to
With reference to
With reference to
With reference to
The various components of the bundle assembly of the invention are advantageously modular in nature for simplicity in assembly. With reference to
It should also be appreciated that a plurality of film generators and arrays of film support structures may be stacked to provide a longer flow path for the polymer melt. With reference to
Although the majority of the examples show three film generators, the actual number required depends on a number of factors. Intermediate film generators are often useful in changing the number of film support structures in successive rows. In order to achieve efficient space utilization, the horizontal spacing within a row of film support structures can be adapted to the melt viscosity of the liquid (i.e., polymer melt). Thus, as the viscosity increases from the top to the bottom of the reactor, the minimum horizontal spacing increases between the adjacent film support structures. Typically as a result, the number of film support structures in a row decreases. Intermediate film generators also facilitate changing the orientation of the film support structures, for example, having the film support structures in successive rows rotated 90 degrees about the reactor centerline.
In another embodiment of the present invention, a polymerization reactor that utilizes the bundle assembly set forth above is provided. With reference to
The film support structures are mounted in the vessel to provide retention of the polymer melts, thereby increasing liquid residence time within the reactor and its exposure to the reaction conditions. The liquid residence time is required to allow sufficient time for the polymerization kinetics to keep up with the enhanced by-product liberation rates achieved by the increase in the liquid-vapor surface area and the enhancement of its renewal. Not only does this design provide more free surface area for vapor to leave the polymer, but it also provides more parallel flow paths so that the thickness of the films are reduced when compared to the prior art such as roof-and-trough trays.
In yet another embodiment of the invention, a method of increasing the degree of polymerization in a polymeric melt using the bundle assembly set forth above is provided. The method of the invention comprises introducing the polymeric melt into a bundle assembly at a sufficient temperature and pressure. The details of the bundle assembly are set forth above. The method of this embodiment comprises contacting the highest film generator and then the highest positioned row of film support structures with the polymeric melt. Next, the optional intermediate film generators and rows of film support structures are contacted with the polymeric melt. Finally, the lowest positioned row of film support structures is contacted with the polymeric melt. After passing over the lowest positioned row of film support structures, the polymeric melt falls from the bundle assembly. The polymeric melt removed from the bundle assembly advantageously has a higher degree of polymerization than when the polymeric melt was introduced into the bundle assembly. In one variation of this embodiment, the reaction temperature is from about 250° C. to about 320° C. and the reaction pressure is from about 0.2 torr to about 30 torr.
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.
