Abstract:
A polymerization reactor comprising one or more circulation loops with one or more inlets for raw material, one or more outlets, and a circulation pump for circulating a reactor charge within the circulation loop. A by-pass line for by-passing the circulation pump connects a point of the loop upstream of the pump with a point downstream of the pump.

Description:
RELATED APPLICATIONS 
       [0001]    This patent application is a continuation of application Ser. No. 12/160,716, filed on Jul. 11, 2008, which is a National Stage Entry of PCT/EP07/50159, filed on Jan. 9, 2007 and claims foreign priority to EP 06100328.1, filed on Jan. 13, 2006, the contents of each are hereby incorporated by reference in their entirety. 
     
    
     FIELD 
       [0002]    The present invention relates to a polymerization reactor comprising one or more circulation loops with one or more inlets for raw material, one or more outlets, and a circulation pump for circulating a reactor charge within the circulation loop. 
       BACKGROUND 
       [0003]    WO 00/07177 discloses a loop reactor for emulsion polymerization. The loop reactor comprises a circulation pump and a tubular circulation loop connecting the pump&#39;s outlet to its inlet. Water, monomers, and stabilizers are continuously fed to the loop and circulated and polymer emulsion is continuously drawn off. The reactor is particularly suitable for the production of polymers derived from vinyl and/or acrylic monomers, used for instance in paints or adhesives. 
         [0004]    A problem encountered in polymerization processes employing a tubular reactor is the formation of deposits from the reaction products on the internal wall of the reactor. These deposits lead to a need for an increased delivery pressure from the circulation pump and impair heat transfer from the reaction medium to, e.g., a coolant in a jacket surrounding the reactor tube, thus leading to higher and often deleterious reactor temperatures or else necessitating an increased coolant circulation rate, a lower coolant temperature, or a reduced rate of production. Fouling also reduces the reactor volume, increasing both the recycle rate and the shear on the emulsion. This shifts the process conditions, which may have been optimized on a clean reactor. In any case, product properties will drift, nullifying the advantages of consistency of production expected from continuous reactors. 
         [0005]    In WO 00/07177 cleaning pigs are used for cleaning the inside of the reactor tubes. The cleaning pigs have a diameter which is about the inner diameter of the reactor tube. The pigs are launched from a pig station and propelled through the loop by the polymerizing emulsion. Since a pig cannot be permitted to pass through the circulation pump, a by-pass line is provided to by-pass the pigs around this pump. 
         [0006]    To clean the parts which cannot be pigged, such as the circulation pump, the reactor is rinsed with cleaning solvent on a regular base. To this end the complete reactor has to be emptied first. Although the solvent cleaning only serves to clean the unpigged section of the loop, the complete loop is filled with cleaning solvent. After cleaning, the solvent is removed and the complete reactor needs to be recharged. This procedure leads to substantial loss of productive time and to high economical and environmental costs. 
         [0007]    It would be advantageous to provide a loop reactor which can be cleaned more efficiently using less cleaning solvent. 
       SUMMARY 
       [0008]    The object of the invention is achieved by providing a polymerization reactor according to the opening paragraph having a by-pass line by-passing the circulation pump, which connects a point of the loop upstream with a point of the loop downstream, both points being provided with a three-way valve, to form a short loop comprising the pump, a cleaning solvent inlet and a cleaning solvent discharge. This creates the possibility to short-circuit the pump section, closing off the main coil. Cleaning solvent can be pumped around in the isolated pump section to clean the pump. Due to the fact that now only the short circuited pump section is solvent cleaned, instead of the complete loop, the amount of cleaning solvent used can be reduced dramatically by more than 90%. Moreover, the main coil does not need to be emptied anymore. 
         [0009]    To aid solubilization of the polymer residues in the section to be cleaned, the section and/or the by-pass line can be provided with jackets connected to a heating medium source, such as heated water, to heat the solvent. 
         [0010]    The solvent can for example be re-circulated for about 15 to 45 minutes before it is pumped out of the equipment. Optionally the circuit is refilled for a second wash or alternatively a small bleed of solvent is continuously pumped into the circuit during the cleaning cycle, such that used solvent overflows to a bin or the like. 
         [0011]    Some typical monomers suitable for use in the present polymerization process include, e.g., butyl acrylate, methyl methacrylate, styrene, vinyl acetate, Veova® 9, Veova® 10, (each ex Shell), ethyl acrylate, 2-ethyl hexyl acrylate, ethylene, and vinyl chloride. The addition reaction is initiated by radicals to give a dispersion of high molecular weight polymer particles, usually of 50 to 3,000 rim diameter, suspended in a medium in which the polymer is insoluble, usually water. Common free radical generators include the sodium, potassium, and ammonium salts of peroxodisulphuric acid, e.g., ammonium peroxodisulphate. Alternatively, redox couples can be used. These consist of an oxidizing agent and a reducing agent. Commonly used oxidizers are the salts of peroxodisulphuric acid and t-butyl hydroperoxide and hydrogen peroxide itself. 
         [0012]    Reducers are sodium sulphite, sodium metabisulphite, sodium formaldehyde sulphoxylate, and sodium dithionate. 
         [0013]    Polymerization of monomers can take place in aqueous suspension and, in that case, raw materials are preferably provided by separate feed streams. These streams introduce fresh monomer and an aqueous solution of stabilizers known as the water phase or, e.g., a pre-emulsion of monomer and water and an aqueous solution in a separate small stream. At the start of the reaction the reactor is filled with water phase made up in a solution tank. Other fillings are also possible, particularly finished emulsion polymer (of the same or a different composition) from a previous run, optionally diluted to any concentration. 
         [0014]    Agitation in the reactor is provided by virtue of the in-line circulation pump. Shortly after the feed streams start to flow, the monomers begin to react and heat is generated. The temperature is stabilized by cooling means, usually by controlled circulation of a cooling fluid (e.g. water) through a cooling jacket. The product flows to the cooling tank, where residual monomer converts to polymer. 
         [0015]    After cooling, the emulsion polymer is filtered to remove any oversize particles or gritty material in the strainer and transferred to the product storage tank. 
         [0016]    Optionally, the polymerization process may be carried out under pressure, for instance under a pressure of 10 to 150 bar. Alternatively, the polymerization may be carried out at ambient pressure. 
         [0017]    Suitable circulation pumps are for instance positive displacement pumps or centrifugal pumps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The forms disclosed herein are illustrated by way of example, and not by way of limitation, in the figure of the accompanying drawing and in which: 
           [0019]      FIG. 1  shows the pump section of a loop reactor  1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Various aspects will now be described with reference to specific forms selected for purposes of illustration. It will be appreciated that the spirit and scope of the apparatus, system and methods disclosed herein are not limited to the selected forms. Moreover, it is to be noted that the figure provided herein is not drawn to any particular proportion or scale, and that many variations can be made to the illustrated form. Reference is now made to  FIG. 1 , wherein like numerals are used to designate like elements throughout. 
         [0021]      FIG. 1  shows the pump section of a loop reactor  1 , having two outer ends  2 ,  3  connected to the outer ends  4 ,  5  of a tubular coiled loop (not shown). The pump section  1  comprises a monomer inlet  6 , a water phase inlet  7 , and an outlet  8  for finished product. A circulation pump  9  serves as a driving means for circulating a reactor charge within the circulation loop. A by-pass line  10  for by-passing the circulation pump  9  connects a point  11  of the loop upstream of the pump  9  with a point  12  downstream of the pump  9 . Both points  11 ,  12  are provided with a three-way valve  13 ,  14 . A second by-pass line  15  includes a pig station  16  to store one or more pigs at rest. The pig station  16  can be isolated using valves. Downstream of the valve  19 , the by-pass line  15  makes a U-turn, the return line being hidden in the side view of the drawing. The second by-pass line  15  returns to the main line just upstream of the valve  14 . Downstream of the U-turn is the outlet  8 . A vent line  20  connects the pig station  16  to the outlet line  8 . The vent line  20  serves to bring the pig to the rest position in the pig station  16  after returning from the coil. The outlet line  8  is provided with a valve  21  just upstream of the junction with the vent line  20 . The vent line  20  is provided with valve  22 . 
         [0022]    To clean the circulation pump  9 , the valves  13 ,  14  are used to close off the main coil and to open the by-pass line  10 . Polymer emulsion contained within the isolated section encompassed by the actuation of the valves  13  and  14  is drained off via a valve  23  and a drain-off line  24  situated underneath the pipe running between the circulation pump  9  and the three-way valve  13 . 
         [0023]    Valves  18  and  22  are opened and valve  21  is closed. Alternatively, valve  21  is left open and valves  18  and  22  remain closed. Either of these two valve options provides an exit route for the solvent. The solvent may move up the vent line  20  to the outlet line  8  or more simply pass vertically up the line  8  through the valve  21 . Above the valve  21  there is a pipe coupling  25  and immediately after this coupling  25  there is a three-way valve (not shown) which is used to transfer the solvent to a small bore line and which leads to a waste solvent bin. High temperature boiling solvent is pumped into the cleaning circuit from a solvent supply line  26  via the circulation pump  9 . Air trapped inside the circuit is bled via a small valve  27  on the by-pass line  10  near the valve  14 . The circulation pump  9  is then set running to provide a solvent circulation. The circulation aids the solubilization of any polymer deposits. Jackets  28  on the pipes are heated with hot water, which results in an elevated solvent temperature and this too aids solubilization. After a period of time of typically  15  to  45  minutes the solution is pumped out of the equipment via the valve  23 . Optionally the circuit is refilled for a second wash or alternatively a small bleed of solvent is continuously pumped into the circuit during the cleaning cycle, such that it overflows to a bin or the like. When all the solution has been drained off, the circuit is filled with water phase, the valves  13 ,  14 ,  18  and  22  and/or  21  are returned to their original positions, after which production can be restarted. 
         [0024]    While the present invention has been described and illustrated by reference to particular forms, those of ordinary skill in the art will appreciate that the invention lends itself to variations not necessarily illustrated herein. For this reason, then, reference should be made solely to the appended claims for purposes of determining the true scope of the present invention.