Abstract:
Both a system and method for cleaning a low pressure separation vessel of a high pressure polyethylene polymerization plant are provided. The system includes a polytetrafluoroethylene lining that covers the interior surfaces of the vessel, and a cover mounting assembly including an annular clamp for detachably mounting a cover over the vessel. The mounting assembly includes a clamp actuator for quickly securing and releasing the cover with respect to a top rim of the vessel. The vessel is drained of liquid polyethylene and allowed to cool to ambient temperature, thus creating a frozen “skin” of polyethylene around the interior surfaces of the vessel. The clamp actuator releases the cover. The polyethylene skin is peeled off the interior sides the vessel and gathered up at the top to form a neck, thus peeling the polyethylene skin away from the polytetrafluoroethylene lining along with any degraded polymers or other impurities that have accumulated on the interior surfaces of the vessel.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention generally relates to a high pressure polymerization process for the manufacture of polyethylene, and is specifically concerned with a system and method for cleaning a low pressure separation vessel of a polyethylene polymerization plant that provides rapid and effective in situ removal of impurities that accumulate within the vessel. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the manufacture of ethylene polymers, ethylene gas is compressed into a supercritical fluid and then heated. The hot supercritical ethylene is then admitted into a tubular polymerization reactor, along with a supply of a chemical initiator and a modifier. The chemical initiator initiates polymerization of the free radical ethylene, while the modifier controls the molecular weight of the resulting polyethylene. Since only about 40% of the ethylene monomers react, the resulting polyethylene product that is discharged from the reactor is a mixture of ethylene polymers intermixed with unreacted ethylene. Consequently, it is necessary to separate the polymers from the ethylene. To this end, a high pressure separator vessel and a low pressure separator vessel are serially connected to the outlet of the polymerization reactor. The high pressure separator vessel initially receives the reactor product from the reactor at about 40,000 psi. The reactor contents are depressurized to about 4000 psi through a control valve into the high pressure separator vessel, which separates most of the polymer from the ethylene. The resulting polyethylene product still contains about 10% unreacted ethylene, and is admitted to the low pressure separator vessel. The lower pressure in this vessel results in the flashing away of the remainder of the unreacted ethylene from the product. The resulting polyethylene is then admitted into an extruder for final processing. 
         [0003]    During processing, the outer walls of the low pressure separator vessel are continuously heated by means of a steam jacket in order to maintain the polyethylene product in a flowable liquid state. The applicants have observed that the non-Newtonian characteristics of the liquid polyethylene flowing through the low pressure separator vessel results in a very slow flow rate at the interface between the liquid polyethylene and inner surface of the vessel. The inner surface of the vessel is also where the interior temperature of the vessel is highest due to its closeness to the steam jacket that surrounds the exterior of the vessel. The combination of the high temperature of the vessel inner surface and the long residence time of the liquid polyethylene over it results in the production of degraded polymers on the inner surface due to thermally-induced, cross-linking reactions. If these degraded polymers are not periodically removed from the inner surfaces of the low pressure separator vessel, they can contaminate the final polyethylene product and degrade its appearance and film properties. The problem is worse in situations where a high clarity and purity polyethylene product is essential for the rendering of a particular final product, such as blown film products, medical applications and sensitive electrical applications. 
         [0004]    To solve this problem, polyethylene manufacturers typically periodically clean the inner walls of the low pressure separator vessel by hydroblasting every several months. But because hydroblasting takes several days and must be done with the vessel in a horizontal position, most polyethylene manufactures replace the fouled low pressure separator vessel with a pre-cleaned, substitute separator vessel in order to reduce system downtime. Unfortunately, such a vessel replacement procedure still takes about a day to implement due to the time required to (1) mechanically disconnect all of the interfaces of the fouled vessel with the other components of the polymerization plant, (2) exchange the multi-ton fouled vessel with a multi-ton cleaned vessel and (3) to re-connect all of the interfaces between the clean vessel and the polymerization plant. Moreover, as the vessel weighs one or more tons, the step of exchanging the fouled vessel with a cleaned vessel must be done by way of a slow and delicate crane operation in order to avoid breakage or damage to the valves, pipes and other interface fittings that must be disconnected and reconnected. 
       SUMMARY OF THE INVENTION 
       [0005]    Clearly, there is a need for an improved cleaning technique for a low pressure separator vessel that is faster and that reduces the amount of downtime of the polymerization plant. Ideally, such a technique would be easier and less expensive to implement, and would reduce the amount of downtime for cleaning operations necessary to maintain a high quality polyethylene product. 
         [0006]    The invention is a system and method for cleaning a separation vessel that fulfills all of the aforementioned needs. To this end, the system of the invention generally comprises a low-stick lining, preferably a polytetrafluoroethylene lining, that covers at least a portion of the interior surfaces of the vessel, and a detachable cover mounting assembly including a clamp for detachably mounting a cover over the vessel in a pressure-tight relationship. “Low-stick” for purposes of this specification and appended claims means a lining producing a reduced tendency for polymer product, such as polyethylene or polypropylene, to adhere to a surface so lined as compared to a surface without such low-stick lining. The system may include a layer of metal applied over the interior surfaces of the vessel to provide adhesion between the polytetrafluoroethylene lining and the interior surfaces of the vessel. The vessel lining is preferably a layer (i.e., film) of polytetrafluoroethylene having a dry film thickness that is preferably between about 0.02 and 0.20 mm, and the metal layer is preferably a nickel layer between about 0.050 and 0.150 mm thick. As used herein “dry film thickness” means the thickness of the film after it has thoroughly dried (e.g., after all the solvent has evaporated and the film has cured). The mounting assembly includes a clamp actuator that secures and releases the clamp into and out of a clamping position. The cover and the top rim of the vessel may each include annular flanges which the clamp may capture and pull together when actuated into the clamping position. The clamp actuator may include one or more hydraulic cylinders that can rapidly secure and release the clamp into and out of a clamping position. The mounting assembly preferably further includes a gasket that provides a pressure-tight seal between the cover and the vessel when the clamp is secured by the clamp actuator. The system may further include a hoist for removing an impurity-laden layer of polyethylene skin off of the polytetrafluoroethylene lining that covers the interior surfaces of the vessel, and a scraping tool, such as a wooden spatula, for initiating the peeling of a polyethylene skin off of the interior sides of the vessel. 
         [0007]    In the method of the invention, the low pressure vessel is emptied of liquid polyethylene and the pressurized ethylene gas is bled off and recycled into the reactor until ambient pressure is achieved. The vessel is then cooled through, e.g., exposure to the environment or, preferably, by circulating cooling water through the heat exchanger panels on the vessel exterior, to substantially ambient temperature, which freezes the liquid polyethylene clinging to the interior surfaces of the vessel into a skin of solid polyethylene. The clamp is then detached from the cover, which allows the cover to be quickly removed from the top of the vessel. The polyethylene skin is next peeled off of the sides of the low pressure vessel and gathered up at the top to form a neck, which in turn is connected to the hoist of the system. The hoist lifts the knot upwardly, which peels the polyethylene skin away from the polytetrafluoroethylene lining along with any impurities that have accumulated on the interior surfaces of the vessel. The hoist lifts the resultant bag-like polyethylene skin completely out of the vessel, thereby completing the cleaning of the vessel. The cover is then re-positioned over the top end of the vessel, and the clamp is re-attached over the cover and the upper rim of the vessel to create a pressure-tight seal between the cover and the vessel. The reactor is re-activated and the vessel is put back into production. 
         [0008]    The cleaning process of the invention requires only about 2 hours to perform, in contrast to the full day required by the prior art method. Moreover, the invention obviates the need for two separate low pressure separator vessels, and does not require disconnection and lifting and lowering steps that can damage the vessel. Finally, the polytetrafluoroethylene lining that covers the interior surfaces of the vessel and, optionally, the cover, not only results in more thorough cleaning when the polyethylene skin is lifted off of the interior surfaces, but also promotes a higher degree of flow on the inner surfaces of the vessel during the manufacture of the polyethylene, thus reducing the number of vessel cleanings required to maintain a high quality product. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic drawing of a polyethylene plant having a low pressure separator vessel that the system of the invention is applied to; 
           [0010]      FIG. 2  is an enlarged side view of the low pressure separator vessel shown in  FIG. 1 ; 
           [0011]      FIG. 3  is an enlarged view of the cross section of the vessel wall area circled in phantom in  FIG. 2 , illustrating both the polytetrafluoroethylene lining and nickel coating of the system of the invention; 
           [0012]      FIG. 4  is a plan view of the view of the low pressure separator vessel shown in  FIG. 1  with the cover removed, illustrating the cover mounting assembly in a closed state in solid lines and in an open state in phantom lines; 
           [0013]      FIG. 5  is an enlarged view of an end of the cover mounting assembly along the line  5 - 5  in  FIG. 2 , illustrating the relationship between the annular flange that circumscribes the upper rim of the vessel, the clamp of the cover mounting assembly, and the clamp securing bolt; 
           [0014]      FIG. 6A  is a partial, side cross sectional view of the cover mounting assembly shown in an open state without the clamp; 
           [0015]      FIG. 6B  is a partial, side cross sectional view of the cover mounting assembly shown in a closed state without the clamp; 
           [0016]      FIG. 6C  illustrates the cover mounting assembly shown in  FIG. 6B  with the clamp; 
           [0017]      FIG. 7  illustrates the initial steps of the cleaning method of the invention after the cover of the vessel has been removed to provide access to a polyethylene skin that has hardened over the inner surface of the vessel; 
           [0018]      FIG. 8  is a top, cross sectional view of the vessel along the line  8 - 8 , illustrating how the polyethylene skin dimples away from the inner surface of the low pressure separator vessel as a result of the cover being removed; 
           [0019]      FIG. 9  illustrates the cleaning method steps of separating the polyethylene skin from the inner surfaces of the vessel with spatulas and gathering the top of the skin into a neck; and 
           [0020]      FIG. 10  illustrates the final steps of cleaning method of the invention of lifting polyethylene skin from the inner surfaces of the vessel with a hoist. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIG. 1  is a schematic of a polymerization plant  1  of the type that includes the low pressure separator vessel  15  that the cleaning system and method are applied to. The low pressure separator vessel typically operates at a pressure in the range of from 0.1 to 20 barg, more preferably from 0.1 to 5 barg, yet more preferably from 0.1 to 2 barg and especially preferably from 0.1 to 0.9 barg (barg=bar gauge, that is, pressure in excess of atmospheric). The plant  1  includes an ethylene feed line  2  which supplies fresh ethylene to a primary compressor  3 . The ethylene discharged from the primary compressor  3  flows via conduit  4  having a valve  4   a  to a secondary compressor  5 . Also entering the secondary compressor  5  is a stream of fresh modifier(s) and/or optional comonomer(s) and a stream of recycled ethylene. The fresh modifier stream is supplied by a separate modifier pump  6 . The recycled ethylene comes from the high pressure recycle system  7 . 
         [0022]    The secondary compressor  5  discharges compressed ethylene in five streams  8   a ,  8   b ,  8   c ,  8   d , and  8   e . Stream  8   a  accounts for 20% of the total ethylene flow. Stream  8   a  is heated by a steam jacket (not shown) which heats the ethylene, prior to entry into the front end of the tubular reactor  9 . The four remaining ethylene side streams  8   b ,  8   c ,  8   d , and  8   e  each enter the reactor as sidestreams. Sidestreams  8   b ,  8   c ,  8   d , and  8   e  are cooled. The tubular reactor  9  is also shown with six initiator inlets  10   a  to  10   f  which are spaced at intervals along reactor  9  and are fed from an initiator mixing and pumping station  11 . 
         [0023]    Downstream of the sixth initiator inlet  10   f  and the sixth reaction zone, the tubular reactor terminates in a high-pressure, let-down valve  12 . The high-pressure, let-down valve  12  controls the pressure in the tubular reactor  9 . Immediately downstream of the high-pressure, let-down valve  12  is product cooler  13 . Upon entry to the product cooler  13 , the reaction mixture is in a phase-separated state. It exits into high pressure separator  14 . The overhead gas from the high pressure separator  14  flows into the high pressure recycle system  7  where the unreacted ethylene is cooled and returned to the secondary compressor  5 . 
         [0024]    The polymer product flows from the bottom of the high pressure separator  14  into the low pressure separator  15 , separating almost all of the remaining ethylene from the polymer. That remaining ethylene is transferred either to a flare (not shown) or a purification unit (not shown) or is recycled via the primary compressor  3  from the product separation unit to the secondary compressor. Molten polymer flows from the bottom of the low pressure separator  15  to an extruder (not shown) for extrusion, cooling and pelletizing. 
         [0025]    With reference now to  FIG. 2 , the separator vessel  15  includes a vessel body  17  having a product inlet  19  mounted on its side for receiving the polymer product from the high pressure separator  14 . Vessel body  17  includes a cylindrical section  18   a  that ends in a frustro-conical section  18   b  at its bottom that functions to funnel purified liquid polyethylene into an extruder (not shown). The cylindrical section  18   a  is surrounded by a steam jacket (not shown) that continuously applies heat to the vessel  15  during production to maintain the polyethylene product in liquid form. The vessel  15  further includes a cover  21  that is sealingly mountable over the top rim of the vessel body. Cover  21  includes an overhead gas outlet  23  for conducting pressurized ethylene gas either back to the primary compressor  3  for recycling or to a flare or purification unit. Cover  21  further includes small and large rupture discs  25   a ,  25   b  for relieving smaller or larger excess pressures in order to avoid a catastrophic bursting of the vessel  15 . Finally, the cover  21  includes a nitrogen purge line  27  for replacing air in the vessel with inert nitrogen prior to putting the vessel on-line, thereby avoiding any degradation of the polyethylene product as a result of oxidation. The diameter of the vessel body  17  can range between 5 and 15 feet (1.52 and 4.57 meters), while the length of the vessel body can range between 10 and 40 feet, (3.05 and 12.2 meters). 
         [0026]    With reference now to  FIG. 3 , the walls  30  of both the vessel body  17  and the cover  21  may be formed from a curved plate  31  of either carbon steel or stainless steel. The cleaning system of the invention includes a layer or lining  32  of a chemical having anti-stick characteristics with respect to polyethylene over the inner surface of the walls  30 . Preferably, layer  32  is comprised of polytetrafluoroethylene having a dry film thickness between about 0.02 and 0.20 mm. More preferably, the dry film thickness of the polytetrafluoroethylene layer  32  of the vessel body  17  is between about 0.02 and 0.07 mm, while the dry film thickness of the polytetrafluoroethylene lining  32  of the cover  21  is between about 0.04 and 0.15 mm. The preferred dry film thickness of the polytetrafluoroethylene lining  32  of the cover  21  is greater due to the presence of more tightly curved surfaces than the inner surface of the walls  30  of the vessel body  17 . When the steel plate  31  forming the walls is formed from carbon steel, a layer  34  of a corrosion-resistant metal  37 , such as nickel is applied over the surface of the inner walls  30  to provide a surface that the polytetrafluoroethylene layer  32  can adhere to. Without such a layer  34 , the rust, corrosion and pitting that invariably forms on the surface of carbon steel over time would provide sites where the polytetrafluoroethylene layer  32  would start peeling off of the inner surfaces of the walls  30 . Such a layer  34  of nickel is preferably applied by electrodeposition to a thickness between about 0.050 and 0.150 mm. When the steel plate  31  forming the walls is formed from stainless steel, no layer  34  of a corrosion-resistant metal is necessary, and the polytetrafluoroethylene layer  32  is applied directly over the inner surface of such stainless steel plate with good adherence. The optional inclusion of the layer  34  of a corrosion-resistant metal in the system of the invention advantageously allows the system to be retrofitted onto carbon steel, low pressure separator vessels  15  used in older polyethylene plants. 
         [0027]    With reference to  FIG. 4 , the cleaning system further includes a cover mounting assembly  36  for detachably and sealingly mounting the cover  21  to the upper rim  37  of the vessel body  17 . To this end, the cover mounting assembly  36  includes a clamp  38  formed from a pair of opposing, semicircular clamp members  39   a, b  that are movable into and out of a clamping position by means of a pair of hydraulically-controlled clamp actuators  40   a, b . The semicircular clamp members  39   a  and  39   b  are supported by a pair of brackets  42   a, b  and  42   c, d , respectively. 
         [0028]    Each of the support brackets  42   a, b  and  42   c, d  includes a slot  44  which slidably receives a guide pin  46  connected to one of the clamp members  39   a, b . Each of the clamp actuators  40   a,b  includes a pair of hydraulic pistons  50   a, b  and  50   c, d  for moving the semicircular clamp members  39   a  and  39   b  from a non-clamping position (illustrated in phantom) that allows the cover  21  to be lifted off the rim  37  to a clamping position (illustrated in solid lines) that sealingly mounts the cover  21  over the upper rim  37 . The slidable engagement between the guide pins  46  and the slots  44  in the support brackets  42   a, b  and  42   c, d  confines the movement of the semicircular clamp members  39   a, b  between the positions illustrated in  FIG. 4  when the hydraulic pistons  50   a, b  and  50   c, d  of the clamp actuators  40   a, b  are operated. 
         [0029]    As is illustrated in  FIGS. 5 and 6A , the cover mounting assembly  36  further includes annular flanges  52  and  56  circumscribing the bottom rim of the cover  21  and the top rim  37  of the vessel body  17 , respectively. The top wall  54  of annular flange  52  and the bottom wall  58  of the annular flange  56  are slightly tapered in opposite directions as shown. A ring-shaped gasket  60  is provided between the bottom rim of the cover  21  and the top rim  37  of the vessel body  17 . The lower wall of the annular flange  52  terminates in a circular lip  62  that is complementary in shape to an annular recess  66  present in the upper wall of the annular flange  56 B. When the cover  21  is positioned over the rim  37  of the vessel body  17 , the gasket  60  is seated between the circular lip  62  of the annular flange  52  and the annular recess  66  of the annular flange  56 , as is shown in  FIG. 6B . 
         [0030]      FIG. 6C  illustrates how the clamp  38  of the cover mounting assembly  36  compresses the gasket  60  between the circular lip  62  and the annular recess  66 . Specifically, each of the semicircular clamp members  39   a, b  includes an annular recess  66  having opposing inner side walls  68   a, b  which are slightly tapered at the same angles as the top wall  54  of the annular flange  52  and the bottom wall  58  of the annular flange  56 . Consequently, when the hydraulic pistons  50   a - d  of the cover mounting assembly  36  are actuated to retract the clamp  38  into the clamping position illustrated in  FIG. 4 , the inner side walls  68   a, b  wedgingly engage the top wall  54  and the bottom wall  58  to squeeze the upper and lower annular flanges  54  and  58  together, thereby compressing the gasket  60  into sealing engagement between the circular lip  62  and annular recess  66 . 
         [0031]    Finally, in order to lock the semicircular clamp members  39   a, b  into the clamping position shown in  FIG. 4 , the cover mounting assembly  36  includes a combination of mounting lugs  70   a - d  and locking bolts  72 , as best seen in  FIG. 5 . Each of the locking bolts  72  includes a ring-shaped end  74  pivotally mounted in each of the lugs  70   a  and  70   c , and a threaded end  75  receivable into a recess in each of the lugs  70   b  and  70   d . When the hydraulic pistons  50   a - d  are actuated, both of the locking bolts  72  may be pivoted into the position illustrated in  FIG. 4 . A locking nut  76  may then be screwed over the threaded ends  75  in order to maintain the semicircular clamp members  39   a, b  in the clamping position after the hydraulic pistons are de-actuated. To restore the semicircular clamp members  39   a, b  back into the unclamping position, the opposite procedure is followed with the locking bolts  72  and the hydraulic pistons  50   a - d  are actuated to withdraw the semicircular clamp members  39   a, b  back into the position illustrated in phantom in  FIG. 4 . 
         [0032]    In addition to the previously-described cleaning system, the invention also includes a method for cleaning the low pressure separator vessel  15  of a polyethylene plant  1 . In the first steps of the cleaning method, the plant  1  is shut down, and an isolation valve (not shown) is closed that prevents a further flow of polyethylene product into the inlet  19  of the vessel body. The polypropylene product within the vessel  15  is allowed to drain out of the frustro-conical section  18   b  at its bottom into the extruder. 
         [0033]    While the vast majority of the polyethylene product will exit the vessel during the drainage step, some of the product will cling to the inner walls  30  of both the vessel body  17  and the cover  21  due the previously described non-Newtonian flow characteristics of the liquid polypropylene. After the drainage step is completed, another valve (also not shown) is opened to vent the overhead gas outlet  23  of the cover  21  to atmospheric pressure. At the same time, cold water is circulated through the steam jacket that surrounds the vessel body  17  to “freeze” the remaining polypropylene into a skin layer  80  that covers the polytetrafluoroethylene layer  32  that lines the inner surfaces of the walls  30 . 
         [0034]    After the vessel  15  has cooled to ambient temperature, the nuts  76  of the locking bolts  72  are removed and the bolts  72  are pivoted into the unlocking position illustrated in  FIG. 4 . The hydraulic pistons  50   a - d  of the cover mounting assembly  15  are actuated to withdraw the semicircular clamp members  39   a, b  back into the unclamping position illustrated in phantom in  FIG. 4 . The cover  21  is then removed, as is illustrated in  FIG. 7 , and the skin layer  80  lining the inner surface of the cover  21  is manually peeled away and removed. 
         [0035]    As is best seen in  FIG. 8 , when the cover  21  is removed, the skin  80  (which is between about 1.5 and 3.0 cm thick) is torn long the interface between the cover  21  and the upper rim  37  of the vessel body  17 . The tearing forces create separations  81  between the inner surfaces of the vessel walls  30  and the skin  80 . With reference to  FIGS. 9 and 10 , these separations advantageously create starting points where the skin  80  may be peeled back from the inner surfaces of the vessel walls  30  and gathered at its upper end into a neck  82 . Specifically, spatulas  83  formed by a wedge-like, wooden head  84   a  connected to a long handle  84   b  are inserted into the areas of separation  81  in order to separate the skin  80  from the vessel walls  30  such that the skin  80  forms a continuous mass (e.g., a bag-like structure) that terminates at its upper end in the neck  82 . After the skin separation step has been completed, a noose  85  is lowered by means of a hoist  87  and is secured around the neck  82  of the bag-like structure of skin  80 . The hoist is then raised to completely remove the bag-like structure of skin  80 , which of course includes all of the degraded polymers that have accumulated over the inner surfaces of the vessel body  17  over time. The cover  21  is then re-attached over the top rim  37  of the vessel body  17  by the actuation of the hydraulic pistons  50   a - d , the locking bolts  72  are locked in place, the overhead gas outlet  23  is reconnected to the recycling line shown in  FIG. 1 . Nitrogen is next admitted through the nitrogen purge line  27  to displace atmospheric oxygen out of the vessel  15 . The plant  1  is re-started, and the product inlet  19  is re-opened. 
         [0036]    While the system and method of this invention have each been described with respect to a preferred embodiment, numerous modifications, equivalences and variations of this invention will become evident to persons of skill in the art. All such modifications, equivalences and variations are encompassed within the scope of this invention, which is limited only by the appended claims and their equivalences.