Abstract:
The present invention provides a contractor apparatus and method for removing solvent from a polymer cement. The resulting polymer is substantially free of solvent and exhibits improved oil absorption and lower fines. In one aspect, a contractor is provided including a cylindrical casing having a high pressure section, a convergence section, a high velocity section, a divergence section, and a discharge section. The polymer cement is introduced into the high pressure section to significantly and unexpectedly improve solvent removal. The convergence and divergence sections have cross-sectional areas that correspond to an effective angle from about 4° to about 65°, such as 6°. The polymer cement is mixed with high pressure steam. After converging, the polymer cement forms more uniform droplets due to high shear of steam. In the divergence and discharge sections, the polymer is substantially devolatized.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the invention relate to an improved apparatus and method for removing solvent from polymer cement. More particularly, embodiments of the invention relate to a more efficient apparatus and method for devolatilizing polymer cement. 
   2. Background of the Related Art 
   Subsequent to a solution polymerization process, the polymer is often isolated from the solvent as a polymer crumb. A typical method of isolation for polymers, such as conjugated diene polymers and copolymers, utilizes a high shear mixer whereby the polymer solution or “cement” is combined with high-pressure steam in a mixing zone of a cylindrical tube. The temperature of the steam is above the maximum boiling point of the solvent and below the temperature at which the polymer shows evidence of decomposition under the conditions of high shear contact. The ratio of steam to solution and the residence time in the mixing zone are sufficient to vaporize at least 90% of the solvent. The sheared mixture is then passed into a cyclone separation zone wherein the polymer is separated from the steam and any vaporized solvent. A detailed description of this process can be found in U.S. Pat. No. 3,804,145, entitled “Process For The Isolation And Recovery Of Polymers”, issued on Apr. 16, 1974, which is incorporated by reference herein. 
   A high shear contactor having a central zone with an adjustable flow constrictor mounted therein is also described in &#39;145. The cement is fed through an opening into a high shear, annular space formed by the constrictor within the central zone. The cement is contacted with steam in the annular space where the solvent begins to vaporize. The mixture of steam, vaporized solvent, and polymer then exit the open end of the contactor at near sonic speeds. 
   In contrast, U.S. Pat. No. 3,202,647, entitled “Elastomer Recovery Process”, teaches a process for recovering elastomers from hydrocarbon solutions wherein the steam and polymer cement are mixed together and injected into the bottom of a hot water vessel by a steam jet system. The steam ejector described in &#39;647 is generally in the configuration of a converge-diverge shape such as the construction of a Penberthy steam ejector. The system description taught by U.S. Pat. No. 3,202,647 is also incorporated by reference herein. 
   While the foregoing designs are adequate for separating certain polymer cements from solvent, the designs are less efficient for removal of solvent from high molecular weight block copolymers. In particular, typical contactors, like those described above, result in polymer crumb having high solvent content, poor oil absorption, poor particle size distribution and poor porosity when forming crumb of an elastomeric block polymer having a large hydrogenated block of a conjugated diene and two polystyrene end blocks. 
   In addition to unacceptably high solvent content, typical contactors are inefficient in their use of steam. Steam consumption is a major expense in a commercial polymer finishing operation. To achieve sufficient solvent removal in a typical contactor, a steam to cement weight ratio of about 1.2:1.0 to 1.5:1.0 is required. In other words, for every pound of polymer cement treated in a typical contactor, 1.2 to 1.5 lbs of high pressure steam is consumed. 
   Therefore, there is a need for a method and apparatus for separating a polymer from its solvent which results in more efficient solvent removal. 
   SUMMARY OF THE INVENTION 
   An improved method and apparatus for separating polymer from solvent using high pressure steam is provided. In one aspect, a contactor is provided that includes a cylindrical casing having a high pressure section, a convergence section, a high velocity section, a divergence section, and a discharge section. The contactor also includes an insert having multiple diameters that form various annular regions therein to provide a change in cross-sectional area corresponding to an effective angle from about 4° to about 65°, such as 6°. The polymer cement is introduced into the high pressure section where it mixes with the steam and begins to form into droplets. The convergence and divergence sections are tapered to provide a change in cross-sectional area corresponding to an effective angle from about 4° to about 65°, such as 6°. The high pressure section forms a uniform droplet size and prevents the mixture from flashing or devolatizing prematurely. After the mixture passes through the high velocity section, the mixture reaches a supersonic speed creating a near vacuum in the divergence section, allowing the polymer cement to devolatilize. As the flashing mixture continues to flow through the divergence section and into the discharge section, the flashing solvent is substantially separated from the polymer. 
   In another aspect, a method for separating solvent from a polymer cement in a contactor apparatus is provided. The method introduces high pressure steam and a polymer cement into a first section of a contactor having a substantially constant cross-sectional area where the steam and polymer cement are mixed. The mixture then flows through a second section having a converging cross-sectional area corresponding to an effective angle of convergence from about 40° to about 65°, such as 6°. The mixture then flows through a third section having a substantially constant cross-sectional area followed by a fourth section having a diverging cross-sectional area corresponding to an effective angle of divergence from about 40° to about 65°, such as 6°. The solvent is flashed from the polymer in the fourth section and the mixture flows through a discharge section having a substantially constant cross-sectional area before recovering a polymer substantially free of the solvent. 
   In yet another aspect, a method for separating solvent from a polymer cement is provided. The method includes separating residual solvent from the polymer cement within a contactor; and devolatizing the polymer cement within a hot water coagulator. The contactor having a first section having an inlet for the polymer cement; a second section having a converging cross-sectional area that corresponds to an effective angle of convergence between about 4° and about 65°; a third section having a smaller cross-sectional area in comparison to the first section; a fourth section having a diverging cross-sectional area that corresponds to an effective angle of divergence between about 4° and about 65°; and a discharge section having a larger cross-sectional area in comparison to the third section. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawing. It is to be noted, however, that the appended drawing illustrates only typical embodiments of this invention and is therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a cross section view of a contactor according to embodiments of the invention described below. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a cross section view of a contactor  300  according to the present invention. The contactor  300  includes a tubular casing having a first portion  310 , a second portion  312 , and a third portion  314 . The contactor  300  further includes an insert  308  positioned within the casing  301 . The insert  308  has a first portion  326 , a second portion  327 , and a third portion  328 . The insert  308  can either extend through an end wall  303  of the contactor  300  near a steam inlet  305  as shown, or can be held in place with one or more spacers (not shown). 
   The second portion of the casing  312  and the second portion of the insert  327  each have an increasing outer diameter. The first portion of the casing  310  and the first portion of the insert  326  each have a constant outer diameter. Similarly, the third portion of the casing  314  and the third portion of the insert  327  have a constant outer diameter. 
   The insert  308  and the surrounding casing  301  form an annulus  302  defined by an inner wall  304  of the casing  301  and an outer wall  306  of the insert  308 . The annulus  302  includes five sections that correspond to the changing diameters of the casing  301  and the insert  308  described above. More particularly, the annulus  302  includes a high pressure section  350 ; a convergence section  355 ; a high velocity section  360 ; a divergence section  365 ; and a discharge section  370 . 
   The high pressure section  350  is formed within the annular space between the constant inner diameter of the first portion  310  of the casing  301  and the constant outer diameter of the first portion  326  of the insert  308 , and has a substantially constant cross-sectional area. The term “substantially constant” as used herein, refers to an effective angle of convergence or divergence of the cross-sectional area less than about 4°. 
   The convergence section  355  is formed within the annular space between the constant inner diameter of the first portion  310  of the casing  301  and the increasing outer diameter of the second portion  327  of the insert  308 . The convergence section  355  has a converging cross-sectional area with an effective angle of convergence from about 4° to about 65°, such as from about 6°. 
   The high shear section  360  is formed within the annular space between the constant inner diameter of the first portion  310  of the casing  301  and the constant outer diameter of the third portion  328  of the insert  308 . The high shear section  360  has a substantially constant cross-sectional area. 
   The divergence section  365  is formed within the annular space between the increasing inner diameter of the second portion  312  of the casing  301  and the constant outer diameter of the third portion  328  of the insert  308 . The divergence section  365  has a diverging cross-sectional area with an effective angle of divergence from about 4° to about 65°, such as about 6°. 
   The discharge section  370  is formed within the annular space between the constant inner diameter of the third portion  314  of the casing  301  and the constant outer diameter third portion  328  of the insert  308 . The discharge section  370  also has a substantially constant cross sectional area. 
   The high pressure section  350  includes a cement entry port  336  where the polymer cement is fed into the contactor  300 . The cement entry port  336  is rounded, squared, slotted, or has any other cross-sectional geometry, and is located anywhere along the first section  350 . In one aspect, the cement entry port is located from about 4 inches to about 7 inches from the start of the convergence section  355 . In another aspect, the inlet port  336  introduces the polymer cement into an annular space  338  that directs the polymer cement through a slot  340  around the casing  301 , as shown in FIG.  1 . The width of the slot  340  can be adjusted to provide the desired pressure drop of from about 10 psi to about 60 psi. 
   The high velocity section  360  has a ratio of length to diameter ranging from about 8 to about 12. The high velocity section  360  provides a flow restriction/constriction to achieve sonic velocity at an end thereof so that a super sonic velocity is achieved at a beginning of the divergence section  365 . The ratio of the cross-sectional area of the high pressure section  350  to the cross sectional area of the high velocity section ranges from about 5 to about 7. The ratio of the cross-sectional area of the discharge section  370  to the cross-sectional area of the high velocity section  360  ranges from about 15 to about 30. 
   The convergence section  355  has a length  380  and a decreasing inner diameter  381 . The decreasing inner diameter  381  provides a decreasing cross-sectional area that corresponds to an effective angle of convergence from about 4° to about 65°, such as about 6°. 
   The divergence section  365  has a length  390  and an inner increasing diameter  391 . The increasing inner diameter  391  provides an increasing cross-sectional area that corresponds to an effective angle of divergence from about 4° to about 65°, such as about 6°. 
     FIG. 1  also shows a hot water coagulator  200  in fluid communication with the contactor. An exemplary hot water coagulator  200  is shown and described in U.S. Pat. No. 3,202,647, entitled “Elastomer Recovery Process”, which was discussed above and incorporated by reference herein. In operation, high-pressure steam flows through the first portion  310  of the casing  301  while a polymer cement is fed through the inlet port  336 . The pressure of the steam at the contactor  300  is 100 psig to 450 psig, such as 150 psig to 350 psig. Accordingly, the temperature of the steam at the contactor  300  is between about 335° F. and about 550° F., such as between about 365° F. and about 550° F., and such as between about 400° and 550° F. The pressure drop across the inlet port  336  is designed to be in a range from about 10 psi to about 60 psi to control the initial cement drop size. 
   The cement concentration may vary from about 5 percent polymer to about 60 percent polymer by weight. The cements may also vary from about 5 percent polymer to about 25 percent polymer by weight. The cement concentration may further vary from about 10 percent polymer to about 20 percent polymer by weight. 
   Within the high pressure  350  and convergence sections  355 , the steam and cement are mixed and intimately contacted. The ratio of steam to cement which enters down-stream processing equipment via the high shear mixer may vary from about 0.3:1.0 to about 1.5:1.0. The lower limit is determined by the problem of obtaining discrete particles. The maximum ratio is determined by economics and the ability of the down-stream processing equipment to remove the solvent vapor and steam. At steam to cement ratios substantially lower than 0.3:1.0, the polymer no longer forms discrete particles but forms large agglomerates. The higher the steam to cement ratio in the contactor  300 , the smaller the particle size. This size is somewhat dependent on polymer/solvent type, cement concentration and steam temperature. Acceptable particle sizes have been achieved at steam/cement ratios from about 0.3:1.0 to about 1.5:1.0, such as between about 0.5:1.0 and about 1.5:1.0, and such as between about 0.5:1.0 and about 0.8:1.0. 
   As the cement and steam are mixed, solvent droplets begin to form due to the shearing effect from the high-speed steam. As the mixture flows into the high velocity section  360 , the cement droplets are broken up and a relatively uniform distribution of droplets is established. The material accelerates to supersonic speed as it flows through the high velocity section  360  and enters the divergence section  365 . Due to the sudden enlargement of volume within the divergence section  365 , a near vacuum is created by pressure differential. This sudden pressure drop results in a rapid de-volatilization of the solvent, and a sufficient separation of the flashing solvent from the polymer crumb. The separated solvent and polymer crumb then flow through the coagulator  200  for further devolitization. 
   This polymer recovery method is useable with any polymer/solvent cement system that can withstand the high temperature steam without decomposing or cross-linking. It is especially good with polyolefin/hydrocarbon cements, polyalkenyl aromatic polymers/inert solvent cements, polyconjugated diene polymer/hydrocarbon cements, copolymers and block-polymers of conjugated diene and alkenyl aromatic hydrocarbons in inert solvents and the hydrogenated and partially hydrogentated derivatives of the above co-polymers and block polymers in inert solvents. The cements are the two and multiblock alpha alkenyl aromatic hydrocarbon/conjugated diene polymers and selectively or totally hydrogenated derivatives of said block polymers dissolved in hydrocarbon solvents having relatively low boiling points such as alkenes, alkanes, arenes, cycloalkenes, or cycloalkanes. These include for example, mixed pentenes, mixed pentanes, cyclohexane, toluene, and mixtures thereof, the only criterion being that the solvent employed in the apparatus and process of the invention have a maximum boiling point such that it is readily vaporized upon contact with steam of a given temperature. Exemplary cements are the polystyrene/polybutadiene, polystyrene/polyisoprene, polystyrene/polybutadiene/polystyrene, polystyrene/polyisoprene/polystyrene block copolymers, or their hydrogenated or partially hydrogenated derivatives. 
   The contactor geometry allows a certain residence time at a high shear rate to produce a polymer product substantially free of residual solvent and water. The lower the water level, the more easily the polymer is dried. Water is produced by the condensation of steam used to flash the solvent from the cement. The low residual solvent means that the polymer is less sticky, thus enabling a dry handling method. The high shear annular space in the high velocity sections as well as the length of the diverging sections determine the shear rate and the residence time. As the length of the diverging sections is increased, the residence time under shearing conditions is also increased thus allowing more time for the cement de-volatization. 
   The contactor geometry also minimizes steam consumption without adversely affecting the quality of the polymer product. Steam represents a large expense in any de-volatilization process. In prior art contactors, the relation of steam to cement has been about 1.2 to 1.5 pounds of steam for every pound of cement, producing a polymer having low fines with an oil absorption rate (OAR) less than 58. The apparatus described herein devolatizes polymer cement in a near vacuum; therefore, the apparatus consumes considerably less steam without sacrificing or otherwise adversely affecting the quality of the product. For example, the contactor as shown in  FIG. 1  uses steam on the order of 0.6 pounds of steam for every pound of cement, a savings of 50 to 60% compared to the prior art. The resulting polymers also exhibit low fines, similar to the prior art produced polymers, but more significantly, the resulting polymers have an OAR greater than 58. The term “OAR” as used herein refers to the rate of 150 parts of oil absorbed per hundred pounds of rubber (phr) measured after one minute. An OAR greater than 58 corresponds to greater than 96% absorption within one minute. 
   While foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.