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Separation of Polymers from Solution

IP.com Disclosure Number: IPCOM000029007D
Publication Date: 2004-Jun-10
Document File: 2 page(s) / 11K

Publishing Venue

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Abstract

The high temperatures and the associated high pressures for devolatization of polymer solutions could be avoided by heating and flashing the polymer solution in more than one and preferably in two stages. Only part of the solvent is flashed off in the first stage so that the temperature does not drop too low, then the polymer rich solution is heated in another heater before the final flash.

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Separation of Polymers from Solution

The process described below relates to the means of recovering polymers from their solution by heating the solution and flashing off the solvent in two steps.  Many polymers are prepared by polymerizing a monomer in the presence of a solvent under conditions needed for the polymer to remain in solution.  There are various reasons for using solvent in the industrial production of polymers.  For example, in polymerizing ethylene to polyethylene the heat of polymerization is high and the solvent provides a heat sink thus enabling good process control and preventing a runaway reaction.  At any rate, the polymer leaves the reactor as a solution and must be separated from the solvent to be recovered.

The equipment for separating the polymer from the solution should be simple, reliable and inexpensive.  The separation process should not contaminate either the recovered polymer or the solvent with byproducts, decomposition products or other undesirable materials.

There are known processes for recovering polymers from their solution.  If the polymer such as polyethylene is not soluble in the solvent at low temperatures, the solution can be cooled to precipitate the polymer.  Alternately, an addition of a non solvent can also precipitate the polymer.  The precipitated polymer can then be recovered by filtration or centrifugation.  This method is widely used for recovering polymers from their solution on a small scale, in pilot plants and laboratory units, for example.  On a large scale, however, the process is slow, costly and complicated.  It is not used.

Another option is to heat the polymer solution in a heat exchanger and then reduce the pressure in one or more stages.  Upon the pressure reduction, the solvent adiabatically flashes off and the polymer is separated from the solvent vapors in a settling vessel.  Although this process works and is commercially used it has many draw-backs.  In each flashing stage the temperature decreases and the initial solution temperature must be high enough to yield in the last stage molten polymer with a low content of solvent.  The rate of heat transfer in the heat exchanger is low because of the high viscosity of polymer solutions and low thermal conductivity.  Also, in order to obtain the molten polymer in the final flashing step, the initial temperature must be quite high.  Thus the heat exchanger and temperature difference between the wall and process stream must be large.  To achieve high wall temperatures requires expensive and difficult to run heating systems such as Dowtherm® or very high pressure steam.  High wall temperature also requires high process pressures to prevent the solvent from flashing in the heat exchanger and may cause degradation of the product or of the solvent.

US Patent 5,037,955 to Dow describes a multi-section heat exchanger suitable for heating viscous polymer solutions.  The improvement is in controlling each section separat...