Abstract:
A method and apparatus for operating a pressure vessel containing a bed of particulate material comprising substantially leveling the bed and employing a fluid flow distributor above the bed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the operation of a pressure vessel and apparatus for carrying out such operation. 
     2. Description of the Prior Art 
     Although, for sake of clarity and brevity, this invention will be described in respect of the solution polymerization of ethylene, it is to be understood that this invention applies generally to curvilinear pressure vessels that operate at an elevated pressure, e.g., at least about 1,000 psig, and that contain a bed of particulate material through which a process fluid is to flow in a substantially uniform manner. For example, this invention can be applied to adsorbent beds, catalyst beds, and fixed beds such as those used in processes such as polymer formation. 
     Heretofore, linear high density polyethylene (HDPE) has been formed by polymerizing ethylene while dissolved in a solvent such as hexane. The resulting solvent solution also contains a polymerization catalyst such as the combination of TiCl 4  and VOCl 3 . The polymerization reaction is carried out in a single liquid phase containing at least the above components using a series of stirred reactors followed by a tubular (plug flow) reactor. Downstream of the last reactor a catalyst deactivator such as acetylacetone is injected into the solution, and the resulting mixture introduced into an adsorption vessel which is a pressure vessel. In the adsorber catalyst compounds and decomposition components of the deactivator are adsorbed from the single phase solution. The polymerization reaction is carried out at an elevated temperature of from about 150 to about 280 degrees Centigrade (C.) at a pressure of from about 2,000 to about 4,000 psig. Thus, the adsorption step of this process is carried out at a very high pressure, and this requires, for sake of capital costs, an adsorber configuration that is curvilinear, typically spherical. 
     The adsorbent material used in this pressure vessel is typically a particulate material. These particles adsorb from the single phase liquid solution various catalyst moieties such as titanium compounds, vanadium compounds, and by-products of the decomposition of the catalyst deactivator. The adsorbent for the exemplary HDPE process above is typically activated alumina particles such as alumina spheres about 1.7 millimeters in diameter. As these particles adsorb catalyst and deactivator compounds from the single phase liquid passing through the adsorbent bed, they change in color, typically from an initially white color to varying shades of gray, to black, the darker the adsorbent particle, the greater the extent of adsorption of the aforementioned materials by that particle. 
     The particulate adsorbent, when initially loaded into the adsorber, is gravity poured through a nozzle opening in an upper portion of the vessel down into the interior of the vessel, and allowed to pile up therein to a predetermined level. This invariably leaves an adsorbent bed in the vessel with an uneven upper surface, typically an inverted conical surface that rises to a peak approaching, but below, the opening through which it was poured. This conical pile of particulates normally piles up at its natural angle of repose, e.g., about a 30 degree angle from the horizontal for the alumina particles used in an HDPE adsorber. 
     After the conical pile of adsorbant is formed in the vessel, the vessel is put into operation and the high temperature, high pressure, single phase solution aforesaid is passed into the nozzle in the vessel for contact with the adsorbent bed. This nozzle is typically an upstanding conduit whose long axis is substantially vertical. The single phase liquid solution is then passed into the nozzle at an angle that is transverse, e.g., a 90 degree angle, to the long axis of the conduit so that the solution must make a sharp turn downward in order to enter the interior of the vessel where the adsorbent bed lies. 
     In the exemplary HDPE process, as with many other processes, a conventional plug flow reactor is employed upstream of the adsorber to accomplish product uniformity with a uniform residence time distribution for the reactants in that reactor. By “plug flow,” what is meant is substantially uniform fluid velocity distribution across a transverse cross-section of a reactor, and maintenance of that flow as that fluid passes longitudinally through the reactor from its entrance to its exit. This gives all portions of that process fluid essentially uniform residence time in the reactor. This same plug flow concept can be applied to other vessels, including, but not limited to, adsorbent vessels. 
     The curvilinear shape of a high pressure adsorber, the conical shape of the adsorbent bed in the adsorber, and the right angle turn the single phase solution must make after it enters the nozzle of the adsorber, all work against achieving anything like plug flow of the solution through the adsorbent bed. This causes mal-distribution of solution as it passes to and through the bed, which results in channeling of solution through localized portions of the bed. This channeling causes underutilization of the adsorbent throughout substantial volumes of that bed, while other portions, where the channeling occurs, are forced to treat too much solution. The result of channeling can be seen in a used alumina bed height profile wherein some portions (groups) of alumina particles are black, while other groups are still white, indicating no adsorption at all. 
     The HDPE process must be carried out in a single phase solution. If two phases (a polymer rich phase and a solution rich phase) were allowed to form, a phenomenon known in the art as “frosting” or “two-phasing” occurs wherein solid polymer forms in the interior of the reactors and adsorbers, and deposits there. Process conditions such as temperature, pressure, and mass composition of the single phase solution stream can determine whether the stream will stay in the single phase or move toward two-phasing. If two-phasing is allowed to continue unchecked, the vessels in which it is occurring will eventually plug up with solid polyethylene thereby requiring shut down of the plant, and clean up of at least the affected vessels, a costly event in terms of lost production and clean-up costs. 
     Mal-distribution of single phase solution flow through an adsorber bed can cause two-phasing and polymer deposition in the bed due to an undesired change in pressure where the solution channels through the bed. This can lead to plugging of at least sections of the bed, up to, and including, the entire bed if left unchecked. This then necessitates a premature and costly shut down of the adsorber and replacement of the bed with fresh adsorbent. 
     Thus, it is highly desirable to operate an HDPE adsorber in a manner that more closely approaches plug flow through the particulate bed. This invention does just that by attacking both the distribution of the process fluid over the bed, and the configuration of the uneven, upper surface of the bed itself. This premise applies as well to other bed containing pressure vessels such as catalyst containing vessels, and the like. 
     SUMMARY OF THE INVENTION 
     Pursuant to this invention, plug flow of a process fluid through a bed in a pressure vessel is more closely approached by the combination of substantially flattening the upper surface of the bed, and employing a flow distributor in the vicinity where the process fluid enters the vessel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a flow sheet for the HDPE process aforesaid. 
         FIG. 2  shows a flow sheet for the adsorber arrangement for the HDPE process of  FIG. 1 . 
         FIG. 3  shows one of the adsorbers of  FIG. 2  with a particulate bed therein. 
         FIG. 4  shows the flow of process fluid internally of the adsorber of  FIG. 3  that leads up to channeling of process fluid in the bed. 
         FIG. 5  shows the flow of process fluid internally of the adsorber of  FIG. 3  when this invention is employed in that absorber. 
         FIGS. 6 through 13  show various embodiments of flow distributors that can be employed in the practice of this invention. 
         FIG. 14  shows the use a flow redirection member that can be employed in the practice of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an ethylene polymerization process  1  wherein an ethylene monomer stream  2  is compressed at  3  and the compressed product removed into line  4 . Solvent  5  and molecular hydrogen  6  are added to stream  4 . One or more co-monomers  7  can also be added to this stream, if desired. Stream  4  is then heated by heat exchanger  8  to form the desired single phase solution, which is then conducted via line  9  to reactor unit  10 . Unit  10  conventionally contains two continuous, stirred reactors (not shown) working in parallel and both feeding a single, continuous, stirred reactor (not shown), which, in turn, feeds a tubular reactor (not shown). 
     The single phase solution product containing polyethylene formed in reactor unit  10  is passed by way of line  11  to adsorber unit  12 . Acetylacetone is injected (see  FIG. 2 ) upstream of adsorber  12 . The single phase solution minus the catalyst and deactivator materials adsorbed by the alumina bed of unit  12  is passed by way of line  13  to a solvent/polymer separation unit  14 , from which is recovered a polymer product  15  that is then sent on for other processing such as extruding and melt cutting. In unit  14  the single phase solution is depressurized in steps to cause two-phasing so that unreacted monomer and solvent can be recovered for return to the polymerization process (not shown) up stream of reactor unit  10 . 
       FIG. 2  shows unit  12  to comprise two downward flow adsorbers  25  and  26  (insulated or un-insulated) arranged for parallel operation so that one such adsorber can be in operation while the other adsorber is shut down for maintenance, replacement of its adsorbent bed, and the like. The single phase solution in line  11  has added thereto catalyst deactivator  20  to terminate the polymerization reaction, and the resulting single phase solution passed by way of line  22  into either of adsorbers  25  or  26  by way of lines  23  or  24 , respectively. When passing through one of adsorbers  25  or  26 , the single phase solution process fluid contacts and flows through the alumina bed (not shown) inside that adsorber for removal of catalyst and deactivator materials from the process fluid as aforesaid. The process fluid leaving the adsorbent bed is passed by way of either of lines  27  or  28  to line  13  for conduct to unit  14 . 
       FIG. 3  shows that when, for example, adsorber  25  was initially filled with alumina adsorbent  30 , the particulate adsorbent was poured (gravity flow) through upper vessel nozzle  31  onto perforate screen  33 , and allowed to build upwardly from screen  33  to the configuration it naturally forms under its natural angle of repose. This configuration is a bed  32  characterized by an upper surface  35  in the configuration of an inverted conical pile. Surface  35  extends upwardly toward nozzle  31  at the natural angle of repose for the particles that make up bed  32 . Peak  36  of surface  35  of bed  32  approaches nozzle  31 , but is below, and spaced from, the outlet opening  37  of that nozzle. Bed  32  can contain one or more materials, mixed or in layers. 
       FIG. 4  shows adsorber  25  of  FIG. 3  after adsorbant flow  30  is stopped, and process fluid  41  introduced into the interior of vessel  25  when that vessel is put into operation in the polymerization process of  FIG. 1 .  FIG. 4  shows that nozzle  31  is upstanding with its long axis essentially vertical, and that it carries a transversely extending inlet conduit  40  for passing process fluid  41  into nozzle  31 . Process fluid  41  thus enters nozzle  31  at an angle that is transverse (90 degrees in  FIG. 4 ) to the long axis of nozzle  31 . Thus, fluid  41  must impinge on an interior wall of nozzle  31  in order to be redirected downwardly toward nozzle opening  37  and, ultimately, to bed  32 . This causes a mal-distribution of fluid  41  as shown by arrows  42  and  43 , the result being that a majority of fluid  41  flows toward the outer periphery  48  of bed  32 . This result is enhanced by the spherical curvature of the walls of vessel  25 . Thus, fluid  41  is concentrated at outer volumes  46  and  47  of bed  32  thereby channeling most of fluid  41  through these volumes, and leaving the central volume  49  either underutilized or not used at all for adsorption purposes. Channeling of fluid  41  through outer volumes  46  and  47  can cause pressure changes in those volumes sufficient to cause two-phasing of fluid  41  in those volumes. This can cause solid polymer deposition in those volumes which, in turn, can cause new channeling of fluid  41  in other, more inner volumes of bed  32  until bed  32  is essentially plugged, even in central portion  49 , and requires shut down of vessel  25  and replacement of plugged bed  32 . 
     The non-uniform distribution of fluid  41  inside nozzle  31  as shown by arrows  42  and  43 , compounded by the uneven (not flat) configuration of upper surface  35  of bed  32  and the round configuration of vessel  25  all work together to encourage undesired channeling  46  and  47  (and, ultimately, two-phasing) near the outer edge (periphery)  48  of bed  32 . This invention combats this combination of negatives. 
       FIG. 5  shows the arrangement of  FIG. 4  after the implementation of one embodiment within this invention. 
     The first step of this invention is to substantially flatten (level) the uneven upper surface  35  of bed  32  as shown by new upper bed surface  50 . Surface  50  does not have to be exactly or completely flat or level in order to obtain the benefits of this invention. Surface  50  just must be substantially more level so that the configuration of the upper surface of bed  32 , unlike the configuration shown in  FIG. 4 , does not substantially favor the flow of fluid  41  toward the newly formed periphery  51  of bed  32 . 
     Leveling of surface  35  of  FIG. 4  to approach surface  50  can be done in any manner desired. It can be done pneumatically and/or mechanically, or any other way obvious to those skilled in the art. For example, an air stream can be imposed on surface  35 , particularly peak  36  to force particles away from peak  36  to form new periphery  51 . Alternatively, a rotating screed such as that used in finishing a newly poured concrete surface could be imposed on peak  36  to wear down the peak by moving particles outwardly there from to form new periphery  51  that is higher inside vessel  25  than original periphery  48 . 
     The second step of this invention employs a mechanical flow distributor  52  to redirect randomly oriented fluid  41  flows  42  and  43  into more uniformly dispersed flows  53 . Flows  53  are more evenly distributed across the entire upper surface  50  within periphery  51  thereby reducing the tendency of fluid  41  to collect near periphery  51  due to the rounded wall configuration of adsorber  25 . 
     In the embodiment of  FIG. 5  flow distributor  52  is in the configuration of an essentially planar perforate plate  55  supported by rod  54  in or near opening  37 . This is shown in better detail in  FIGS. 6 and 7 . In  FIGS. 6 and 7 , plate  55  is shown to contain a plurality of apertures  60  through the full thickness thereof, and through which fluid  41  can uniformly flow as shown by arrows  53 . In  FIG. 7  plate  55  is shown to be round in its external configuration, but any other configuration, be it square, rectangular, triangular, or the like can be employed so long as uniform distribution of fluid  41  is obtained as shown in  FIG. 5 . Plate  55  can be any thickness and composition so long as it will maintain its configuration under the impingement of fluid  41  and not react chemically with that fluid. The transverse area of plate  55 , as represented by the upper surface  71  of that plate including apertures  60 , can vary widely, but will preferably be not significantly larger than the transverse, cross-sectional area of nozzle opening  37 , and can be smaller than such cross-sectional area of opening  37  so long as a more even distribution of down falling fluid  41  is achieved. 
     It should be noted that rod  54  and plate  55  are essentially fixed in place. Reciprocation or rotation of either element would cause undesired turbulence in the flow of fluid  41 , and detract from achieving the uniform flow achieved by this invention. 
       FIG. 8  shows one of many alternate embodiments that can be used as a flow distributor within this invention. In  FIG. 8 , the flow distributor configuration used is a sphere  80  supported on rod  54 . Sphere  80 , like plate  55  and other embodiments set forth hereinbelow, would be carried in or near, preferably just below nozzle opening  37  as shown in  FIG. 5 , and can be hollow or solid. A hemispherical or “less than spherical” distributor form would also cause undesired turbulence in the flow of fluid  41 , and would not achieve the uniform flow results for fluid  41  of this invention. This premise applies as well to the embodiments of  FIGS. 9-12  below. 
       FIG. 9  shows another distributor embodiment in the form of a lenticular member  90  supported on rod  54  in the same relation to opening  37  (not shown) as shown for sphere  80  of  FIG. 8 . 
       FIG. 10  shows another distributor embodiment in the form of a cube  100  carried by rod  54  with one edge  101  facing opening  37  (not shown) in the same spatial relation to that opening as sphere  80  of  FIG. 8 . 
       FIG. 11  shows a rectangular (rectilinear) form  110  carried by rod  54  with one edge  111  facing opening  37  (not shown) in the same spatial relation to that opening as sphere  80  of  FIG. 8 . 
       FIG. 12  shows yet another distributor in the form of a trapezoid  120  carried on its smaller face  121  by rod  54  so that sloping faces  122  of the trapezoid direct fluid  41  flow outwardly as shown by arrows  123 . 
     To provide for more even distribution with a trapezoidal form, a plurality of hollow trapezoids nested within one another can be employed so that the trapezoidal shaped distributor is, in effect, perforate and performs uniform fluid flow distribution similar to that shown for plate  55  ( FIG. 5 ). This is shown in  FIG. 13  wherein form  120  is shown to be hollow, topless, and bottomless. Trapezoidal faces  122  of form  120  have disposed within the hollow interior of form  120 , nested, smaller, trapezoidal form  130  having faces  134 . Internal faces  134  are carried spaced from rod  54  by means of spaced apart spacers  131  so that fluid can flow between faces  122  and  134  and between adjacent spacers  131 . Similarly, faces  134  and  122  are spaced apart with spacers  132 . Thus, fluid  41  can be evenly distributed over the outside of faces  122  and  134 , and inside faces  134  adjacent rod  54 , all as shown by arrows  133 . All such faces are essentially smooth, as can be the case with the other embodiments here in above. 
       FIG. 14  shows nozzle  31  to carry internally thereof a member  140  that is in fluid communication with conduit  40 , member  140  carrying a downwardly extending, closed portion  141  that carries a plurality of perforations through which fluid  41  can flow. Thus, fluid  41  leaving conduit  40  and entering member  140  is redirected from its transverse flow direction into a new direction that is substantially parallel with the long axis of nozzle  31 . Since end  142  of portion  141  is closed, fluid  41  leaves closed portion  141 , and member  140 , in a redirected direction that is once again substantially transverse to the long axis of nozzle  31  as shown by arrows  143 . Fluid  41  then falls downwardly in nozzle  31 , through opening  37  and, at least in part, on to the top surface of plate  55 . This distributes fluid  41  evenly over the upper surface  50  of bed  32  as shown by arrows  144 .