SEAL FOR ROTATABLE SHAFT

Abstract: A dry seal for a rotatable shaft (36) is disclosed. The seal includes an axially monolithic elastomeric sealing element (11) having a shaft side (12) and a casing side(14). The shaft side is provided with a plurality of radially inwardly extending circumferential fins (20), each fin tapering toward an apex (22). The cross-section of each fin in a plane passing through the axis of the shaft (36) is asymmetric. Two spaced fins with apices pointing away from one another define therebetween a circumferential purge chamber (32). Optionally, adjacent fins having apices pointing in the same axial direction away from the purge chamber define therebetween a circumferential sealing chamber.

Description: SEA FOR ROTATABLE SHAFT Technical Field This invention relates to seal devices, and, in par¬ ticular, to a dry seal about a rotatable shaft. Background of the Invention In rotary pumps, fans, compressors, agitators, etc., a shaft projects through a casing in a region known as the "stuffing box" or "packing box". These terms are derived from the fact that in order to separate two environments, namely, that within the pump, fan, compressor, agitator, etc., and the atmosphere, and to prevent leakage of one environment into the other, material had to be stuffed or packed around the shaft where it passed through its casing. Soft packing materials have been employed for this purpose. Presently, mechanical seals are commonly used for sealing against liquids when using a rotating shaft. These seals consist of radial planar surfaces which are normal to the shaft axis. The surfaces cooperate to func- tion as a bearing. A labyrinth seal is a type of mechanical seal. This seal is not fluid-tight but limits leakage by means of a tortuous path and induced turbulence. The seal consists of a ring or series of rings about the shaft which provide successive expansion of the fluid. As the differential pressure across any individual restriction is small, total leakage is minimized. These rings have a shaft clearance which is constantly maintained. Illustrative of labyrinth seals is British Patent No. 705,444 to Allen et al. When no external leakage is permissible, purging with an inert gas is usually provided. Illustrative of such seals are those shown in U.S. Patent No. 3,071,384 to Fri- berg. Such seals utilize a constant flow of inert gas in combination with a vacuum to prevent leakage along a shaft (Column 5, lines 30-74). This requires a clearance between the spaced sections of the seal and the rotor and stator. Alternatively, a liquid can be used as the purge medium to prevent leakage past the shaft and to provide lubricity. These seals are expensive, however. Further¬ more, these seals are time-consuming to install, thus excessive downtime of the equipment is required when such a seal is to be replaced. They are also undesirable for a number of other reasons. Often, the liquid purge medium either leaks into the environment or the pump, turbine or the like, consequently the environment, the medium being pumped, or the equipment itself becomes contaminated. The liquid purge medium also can, and does, leak into the atmosphere, and may present undesirable safety conditions including potential fire hazards. In addition, many of the prior art seals require the device operated upon by the- shaft be disassembled to replace the seal. This is undesirable because of the additional man-hours and downtime required to disassemble and reassemble the device. Accordingly, it is desirable to provide a seal which is dry, prevents leakage of a fluid along the shaft, and is easy to install. The present invention satisfies these desires. Summary of the Invention The present invention contemplates a seal, for a rotatable shaft, that can be replaced relatively inexpen¬ sively and easily. This seal is monolithic along the shaft and utilizes a plurality of circumferential fins having an asymmetric cross-section to define, together with the shaft, plural sealing chambers and prevent or minimize undesirable leakage. The seal of the present invention is a dry seal which includes a single, elongated axially monolithic elastom- eric sealing element, or a plurality of such sealing ele- ments, each having a shaft side and a casing side. The shaft side is provided with a plurality of radially inwardly extending circumferential fins, each fin tapering towards an apex. The cross-section of each fin in a plane that includes the axis of the shaft is asymmetric. Two spaced fins define therebetween a circumferential purge chamber having at least one inlet which extends through the elastomeric sealing element to the purge chamber. The apices of the fins on each side of the purge chamber point in an axial direction away from the purge chamber. Each fin apex extends to the shaft, with minimal or substan¬ tially no clearance, when the seal is positioned about the shaft. Optionally, a plurality of adjacent fins having apices pointing in the same axial direction away from the purge chamber are provided on each side of the purge cham¬ ber to define therebetween a sealing chamber or chambers. In use, the seal is first positioned about the shaft. Inert gas is then introduced under pressure through the purge chamber inlet into the purge chamber. Inert gas thus flows into and purges any adjacent sealing chambers defined between the fins. In use, the principal flow of the purge gas is toward the interior of the vessel from which the shaft extends. The seal of the present invention is easily installed with a minimum of downtime. Furthermore, inasmuch as the seal is a "dry" seal, the undesirable safety conditions of the prior art liquid purge seals are overcome without the loss of lubricity. In addition, the selection of a seal material such as a fluoroelastomer further assists in min ¬ imizing friction between the seal and the shaft. Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention, the accompanying examples, the drawings, and the appended claims. Brief Description of the Drawings In the drawings: FIGURE 1 is a perspective view in partial section of a seal embodying the present invention; FIGURE 2 is a longitudinal sectional view of a seal embodying the present invention secured to a seal casing and positioned about a rotatable shaft; and FIGURE 3 is an expanded perspective view showing unassembled elements of a split seal embodying the present invention positioned about a rotatable shaft, the shaft being shown partially in phantom. Description of Preferred Embodiments While this invention is susceptible to embodiment in many different forms, there are shown in the drawings and will be described in detail, preferred embodiments of the invention. It should be understood, however, that the present disclosure is to be considered as an exemplifica- tion of the principles of this invention and is not intended to limit the invention to the embodiments illus¬ trated. As used in the ensuing specification and claims, the term "axially monolithic" means that in the axial direc- tion along the shaft there is a single elongated sealing element. However, about the periphery of the shaft the seal may be constituted by one or more of such axially monolithic sealing elements. An embodiment of the present invention, a seal 10 for a rotatable shaft 36, is shown in FIGURES 1 and 2. The seal 10 includes an elastomeric sealing element 11 which is axially as well as circumferentially monolithic, and has a shaft side 12 and a casing side 14. The shaft side 12 is provided with a plurality of radially inwardly extending circumferential fins 20 that define, together with the shaft 36, a circumferential purge chamber 32 and, optionally, one or more circumferential sealing chambers 24 about the shaft 36 on each side of purge chamber 32. Each fin 20 tapers towards an apex 22. The fins 20 have an asymmetric cross-section 21 in a plane that includes the axis of the shaft 36. The angle at which the apices 22 of the fins 20 point in an axial direction away from the purge chamber 32 usually is about 25° to about 40° and preferably is about 30°. The depth of the fins 20 usually is about 0.275 inches to about 0.375 inches, pref¬ erably about 0.3 inches. The spacing between adjacent fins 20 usually is about 0.1 inch to about 0.75 inches, preferably about 0.4 inches. Two spaced fins with apices pointing away from one another define therebetween a circumferential purge cham¬ ber 32 which is in communication with an inert gas source (not shown) via at least one purge chamber inlet 30. Pre¬ ferably, there are provided six purge chamber inlets 30 spaced substantially equally about the purge chamber 32 in a plane perpendicular to the axis of the shaft 36. In a preferred embodiment, a plurality of fins is provided on each side of the purge chamber 32 so that two adjacent fins 20 having apices 22 pointing in the same axial direction away from the purge chamber 32 together define a circumferential sealing chamber 24. Preferably, at least one sealing chamber 24 is adjacent to the purge chamber 32 on each side thereof, and more preferably there are at least three such sealing chambers. The sealing chamber 24 thus defined has an asymmetric cross-section in a plane passing through the axis of the shaft 36 as shown in FIGURE 2. The volume ratio of sealing chamber-to-purge chamber usually is in the range of about 0.25 to about 2. Preferably, the volume of each sealing chamber is about one-half of the volume of the purge chamber; however, the volume of each individual sealing chamber of the seal need not be the same. Apex 22 of each fin 20 extends toward rotatable shaft 36. While the apices 22 need not contact the shaft 36, the clearance between apices 22 and shaft 36 is minimal, i.e., 0.050 inch, or less. This provides improved overall sealing characteristics. Since at most only a relatively small surface area of the seal 10 actually contacts the shaft 36 at a band contact area 34, the total contact sur¬ face between the seal and the shaft is relatively small in any event, thus minimizing friction. Friction can be fur¬ ther reduced by manufacturing the elastomeric sealing ele¬ ment from a filled fluoropolymer having a relatively low coefficient of friction vis-a-vis the shaft. Preferred filled fluoropolymers are carbon-filled fluoropolymers of the type disclosed in U.S. Patent No. 2,891,921 to Kumnick et al. These fluoropolymers are commercially available. An O-ring receiving groove 16 is defined by the elas ¬ tomeric sealing element 11 in one end face thereof. The O-ring 17 received within groove 16 prevents leakage between the seal 10 and the seal casing 26. The elastomeric sealing element 11 is also internally threaded at one end to receive threads 28 of the seal casing 26 for securing the seal 10 to the seal casing 26. FIGURE 3 illustrates an alternative embodiment of the seal 110. In this alternative embodiment, the elastomeric sealing element 111 is generally of the same external and internal configuration as sealing element 11 but is com¬ prised of two complementary, axially monolithic portions 138 and 140 positioned about the shaft 136. Thus, while the sealing element 111 is axially monolithic along the shaft 136, it is not circumferentially monolithic about the shaft 136. The axially monolithic portions 138 and 140 complement one another and completely surround the shaft when installed. In this view, the seal 110 is shown unassembled. Therefore, the apex 122 of fin 120 is not shown in contact with the shaft 136 although in an assem¬ bled seal apex 122 may bear against the shaft 136. Also shown in FIGURE 3 are the purge chamber 132 and the six purge chamber inlets 130 spaced substantially equally about the periphery of purge chamber 132 in a plane perpendicular to the axis of the shaft 136. This arrangement of the purge chamber inlets 130 is the same as the respective arrangement in the embodiment of FIGURES 1 and 2. The use of a preferred embodiment of the present invention will be discussed in connection with FIGURES 1 and 2. The alternative embodiment of FIGURE 3 is used in a like manner but provides additional time savings during installation by virtue of its two complementary portions. The seal 10 is first positioned about shaft 36 and then the threads 28 of the seal casing 26 are matingly engaged with internal threads 18 of seal 10. The O-ring 17 in the O-ring receiving groove 16 is thereby urged into contact with the seal casing 26 to prevent leakage between the seal 10 and the seal casing 26. Inert gas from a source (not shown) is introduced into the circumferential purge chamber 32 through the purge chamber inlet 30. Preferably, six purge chamber inlets 30 are utilized to provide equal distribution of the inert gas. After all sealing chambers 24 are purged by the introduction of inert gas, the inert gas flow is maintained in a direction along the shaft and toward the interior of the vessel from which the shaft extends. The fins 20 in cooperation with the purge chamber 32 and the sealing chambers 24, and coacting with the shaft 36, prevent undesirable leakage of fluid along the shaft 36. Thus, contamination of the environment or equipment is prevented or at least minimized. Because of the shape and orientation of the fins 20, any external pressure exerted thereupon in an axial direction is resisted. The present invention can be molded or machined from a material of construction which is compatible with the operating environment, as desired. Particularly preferred as materials of construction are the aforementioned filled fluoropolymers. The seal of the present invention can be installed with minimal downtime as disassembly of the device oper¬ ated upon by the shaft usually is not required. The foregoing specification is intended as illustra¬ tive and is not to be taken as limiting. Still other var- iations within the spirit and the scope of this invention are possible and will readily present themselves to those skilled in the art. -8- The seals of this invention are suitable for use in the process for preparing aromatic carboxylic acids by the liquid-phase oxidation of alkyl substituted aromatic hydrocarbons. In particular, the seals of this invention are useful in the preparation of terephthalic acid by the liquid-phase oxidation of para-xylene. Terephthalic acid is an important commercial chemical used in the prepara¬ tion of polyester materials as well as other polymeric materials. In the process of producing terephthalic acid from para-xylene relatively pure para-xylene (p-xylene) is required. One particularly suitable method of preparing pure p-xylene comprises the low-temperature crystalliza¬ tion of p-xylene from a mixture of p-xylene, m-xylene and o-xylene followed by a separation of the crystallized p-xylene from the remaining mother liquor by a low-temper¬ ature separation process wherein centrifugation is a pre¬ ferred method for separating the crystallized p-xylene from the liquid mother liquor. The seals of this invention are particularly suitable for use in the centrifuges used for recovering the pure, crystallized p-xylene. In these centrifuges the liquid xylene mother liquor must not escape along the centrifuge shaft and beyond the centrifuge casing. Otherwise, the liquid xylene leakage would present a serious fire hazard in the manufacturing plant. Furthermore, it is essential to minimize the time the centrifuge is non-operational by providing parts for the centrifuge that do not malfunction and by providing parts that do not frequently wear out. The seals of this invention meet all of the aforemen¬ tioned criteria. Due to their labyrinth design, gas purge feature and purge chamber, they prevent the leakage of the flammable xylene mixture along the centrifuge shaft. The seals of this invention are more durable than the conven- tional mechanical seals thereby reducing the need for replacement due to malfunction and wear. Furthermore, due to their design, and particularly the split seal design, -9- the seals of this invention are more easily and more rap¬ idly replaced than the conventional prior art seals. The next step in the preparation of terephthalic acid requires the liquid-phase oxidation of the purified p-xy- lene. In this step, purified p-xylene is oxidized using an oxygen containing gas, such as air, in a liquid medium comprising an aliphatic low molecular weight carboxylic acid solvent, in the presence of a catalyst comprising cobalt, manganese and bromine components, at a temperature of from 100°C to about 350°C, and at a pressure sufficient to maintain at least a portion of the solvent in the liquid phase, to form a reaction product mixture contain¬ ing terephthalic acid. During this liquid-phase oxida¬ tion, the two methyl groups on the p-xylene aromatic nucleus are both oxidized to carboxylic acid groups. In this process acetic acid is a preferred reaction solvent and hydrogen bromide is a preferred source of the bromine catalyst component. The manganese and cobalt catalyst components may be obtained from a variety of sources, how- ever, the acetate salts of these metals are preferred. The liquid-phase oxidation to p-xylene to tere¬ phthalic acid is accomplished in a reactor vessel compris¬ ing an inlet or inlets for air or other oxygen containing gas, baffles along the inside walls of the vessel to pro- mote agitation, and an agitator to vigorously agitate the oxidation reaction mixture during the course of the oxida¬ tion reaction. The agitator comprises a shaft extending through the reactor vessel wall with the end of the shaft located within the vessel having one or more impellers secured to the shaft to effect the agitation of the reaction mixture upon the rotation of the shaft. The other end of the shaft is connected to a means for rapidly rotating the agitator shaft. At the point where the shaft passes through the reactor vessel wall a seal is required to prevent leakage of the gaseous or liquid-phase compo¬ nents of the reaction mixture. The seals of this invention are particularly useful for providing an effec- tive seal between the agitator shaft and the wall of the reactor vessel. The seals of this invention are easily replaced, particularly the split seal of this invention, and they have an exceedingly long useful life. Due to their design, they are not susceptible to frequent mal¬ function. In addition, the elastomeric seal materials are inert to the corrosive oxidation reaction mixture which comprises acetic acid and other acidic components such as hydrogen bromide. The seals are, therefore, not prone to wear or malfunction due to corrosion. Subsequent to the liquid-phase oxidation reaction, the oxidation reaction mixture is cooled, or treated in some other manner, to effect the crystallization of tere¬ phthalic acid from the oxidation reaction mixture thereby producing a slurry of terephthalic acid in the oxidation reaction mother liquor. The crude terephthalic acid is subsequently physically separated from the mother liquor and centrifugation is particularly suitable means for sep¬ arating the crude terephthalic acid from the oxidation reaction mother liquor. The seals of this invention are particularly suitable for use in the centrifuges used to recover the crude tere¬ phthalic acid from the oxidation reaction mother liquor. The seals prevent the leakage of the mother liquor along the seal shaft extending out from the centrifuge casing. The oxidation reaction mother liquor comprises acetic acid and other acidic components, and in order to eliminate a potential hazard leakage from the centrifuge must be pre¬ vented. In addition, the elastomeric materials used for the construction of the seals of this invention are not susceptible to corrosion. The oxidation reaction mother liquor, particularly if held at elevated temperatures during the centrifugation process, is corrosive due to its acidic components. The elastomeric materials used for the seals of this invention are superior to the mechanical seals of the prior art in withstanding the exposure to the corrosive oxidation reaction mother liquor. The seals of this invention when placed on centri¬ fuges used for separating terephthalic acid from oxidation reaction mother liquor are also more durable and more easily replaced than the conventional prior art seals thereby reducing the time the centrifuges are non-opera¬ tional. This results in a significant cost savings to the manufacturing process. The split seals of this invention are particularly preferred since they are most easily replaced. An additional process step in the manufacture of ter ¬ ephthalic acid is a purification step wherein crude tere ¬ phthalic acid is converted to purified terephthalic acid by exposing an aqueous solution of the crude terephthalic acid to hydrogen over a catalyst metal selected from a Group VIII metal of the Periodic Table of Elements. Pref¬ erably, the Group VIII is supported on a support material. The hydrogen treatment is carried out at a temperature of about 100°C to about 350°C and at a pressure sufficient to maintain the solution of crude terephthalic acid substan- tially in the liquid phase. Upon cooling the solution treated in this manner, substantially purified tere¬ phthalic acid crystallizes. The purified terephthalic acid is separated from the water solvent and centrifuga¬ tion is a particularly suitable means for effecting this separation. The seals of this invention are advanta¬ geously employed in the centrifuge apparatus for this pro ¬ cess due to the long service life of the seals and also because they are easily replaced. The split seals are particularly advantageous since they are most easily and most rapidly replaced. While the oxidation processes described above are for the oxidation of p-xylene to terephthalic acid, similar processes are applicable for the liquid-phase oxidation of all alkyl substituted aromatic hydrocarbons as, for exam- pie, the oxidation of meta-xylene to isophthalic acid, the oxidation of o-xylene to phthalic acid or anhydride, the oxidation of pseudocumene to trimellitic acid or anhy- dride, the oxidation of durene to pyromellitic acid or anhydride, of mesitylene to trimesic acid, of 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid, and of other alkyl substituted aromatic compounds wherein the alkyl groups are oxidizable to carboxylic acid groups using liquid-phase oxidation conditions. The seals of this invention are useful in the equipment used in all of these processes. Additionally, the seals of this invention can be used in other machinery or equipment within the plants that manufacture aromatic carboxylic acids wherein the equip¬ ment or machinery contains a rotating shaft and where it is beneficial to provide for a durable, leak-preventing, easily replaceable seal between the rotating shaft and the housing or casing of the equipment or machinery, such as, for example, dryer circulation fans, oxidation process air compressors, rotary pumps and oxidation catalyst recovery centrifuges.

Claims: I CLAIM; 1. A seal for a rotatable shaft comprising: at least one axially monolithic, elastomeric sealing element having a shaft side and a casing side, and provided on the shaft side thereof with a plurality of radially inwardly extending circumferential fins each tapering toward an apex; each of said fins having an asymmetric cross- section in a plane passing through the axis of said shaft; two of said fins being spaced from one another and defining therebetween a circumferential purge chamber; the apex of each of said fins extending toward said shaft when the seal is positioned about the shaft; a plurality of adjacent fins provided on each side of said purge chamber defining therebetween a circum¬ ferential sealing chamber; said adjacent fins having apices pointing in the same axial direction away from said purge chamber and said sealing chamber having an asymmet¬ ric cross-section in a plane passing through the axis of said shaft. 2. The seal of claim 1 further provided with at least one purge chamber inlet defined by and extending through said elastomeric sealing element to said purge chamber. 3. The seal of claim 2 wherein six purge chamber inlets are spaced substantially equally about the periph¬ ery of said purge chamber. 4. The seal of claim 1 wherein said elastomeric sealing element comprises a plurality of complementary portions. 5. The seal of claim 1 wherein said elastomeric sealing element is composed of a filled fluoropolymer. 6. The seal of claim 5 wherein said filled fluoro¬ polymer is a carbon-filled fluoropolymer. 7. The seal of claim 1 wherein said elastomeric sealing element is provided with an O-ring receiving groove at one end face of the sealing element. 8. The seal of claim 1 wherein said elastomeric sealing element is provided with a plurality of circumfer¬ ential internal threads at one end of said elastomeric sealing element, said internal threads being radially inwardly extended on the shaft side of said elastomeric sealing element. 9. A seal for rotatable shaft comprising: at least one axially monolithic, elastomeric sealing element having a shaft side and a casing side, and provided on the shaft side thereof with a plurality of radially inwardly extending circumferential fins each taping toward an apex; each of said fins having an asymmetric cross- section in a plane passing through the axis of said shaft; two of said fins being spaced from one another and defining therebetween a circumferential purge chamber; the apex of each of said fins extending toward said shaft when the seal is positioned about the shaft; a plurality of adjacent fins provided on each side of said purge chamber defining therebetween a circum¬ ferential sealing chamber; said adjacent fins having apices pointing in the same axial direction away from said purge chamber; said sealing chamber having a asymmetric cross- section in a plane passing through the axis of said shaft; said elastomeric sealing element being provided with an O-ring receiving groove at one end face of said elastomeric sealing element; and said elastomeric sealing element being provided with a plurality of circumferential internal threads at that end of said elastomeric sealing element which has said O-ring receiving groove, said internal threads being radially inwardly extended on the shaft side of said elas¬ tomeric sealing element. 10. The seal of claim 1 wherein said elastomeric sealing element exists as two complimentary parts. 11. A process for the preparation of terephthalic acid comprising the steps of: oxidizing para-xylene in the liquid phase with an oxygen containing gas in an aliphatic low molecular weight carboxylic acid solvent in the presence of a catalyst com¬ prising cobalt, manganese and bromine components, at a temperature of from about 100°C to about 350°C, and at a pressure to maintain at least a portion of the solvent in the liquid phase to form a reaction product mixture com- prising terephthalic acid; cooling the product mixture to crystallize at least part of the terephthalic acid; separating the crystallized terephthalic acid from the remaining reaction product mixture using a centrifuge equipped with seals as claimed in Claims 1 through 10. 12. The process of Claim 11 wherein said aliphatic low molecular weight carboxylic acid solvent comprises acetic acid. 13. The process of Claim 11 wherein said bromine component comprises hydrogen bromide. 14. The process of Claim 11 wherein said oxygen con¬ taining gas is air. 15. A process for the purification of crude tere¬ phthalic acid comprising the steps of: passing an aqueous solution of crude terephthalic acid, at a temperature of from about 100°C to about 350°C and at a pressure sufficient to maintain the solution sub¬ stantially in the liquid phase, through a catalyst bed in the presence of hydrogen, said catalyst bed comprising a support material supporting a catalyst metal selected from at least one of the Group VIII metals of the Periodic Table of Elements; cooling the hydrogen-treated aqueous solution to effect a separation of purified terephthalic acid by crys- tallization; separating said purified terephthalic acid from the remaining solution using a centrifuge equipped with seals as claimed in Claims 1 through 10. 16. The process of Claim 15 wherein said catalyst metal is palladium and said support material is carbon. 17. A process for preparing terephthalic acid com¬ prising oxidizing para-xylene in the liquid phase with an oxygen containing gas in a aliphatic low molecular weight carboxylic acid solvent in the presence of a catalyst com- prising cobalt, manganese and bromine components, at a temperature of from about 100°C to about 350°C, and at a pressure to maintain at least a portion of the solvent in the liquid-phase to form terephthalic acid wherein the liquid-phase oxidation is carried out in a reactor vessel comprising at least one inlet for the oxygen containing gas, baffles and an agitator wherein said agitator com¬ prises a shaft with at least one impeller attached thereto and a seal for sealing said shaft to the reactor vessel wherein said seal is a seal as claimed in Claims 1 through 10.

Inventors: RICHARDS HIRAM BRYANT, RICHARDS, HIRAM, BRYANT
Applicants: AMOCO CORP, AMOCO CORPORATION
Application year: 1989
Main classification: F16J15
IPC codes: C07B61, C07C51, C07C63, F16J15