Concentration of viscous solutions

Claim 1

PROCESS FOR CONCENTRATING A VISCOUS SOLUTION OF A POLYMERIC SUBSTANCE OF HIGH MOLECULAR WEIGHT IN A VOLATILE SOLVENT, WHICH COMPRISES HEATING THE SOLUTION TO A TEMPERATURE WHICH IS ABOVE ITS BOILING POINT AT ATMOSPHERIC PRESSURE AND AT WHICH TEMPERATURE THE POLYMERIC SUBSTANCE IS THERMALLY STABLE, THE PRESSURE UNDER WHICH THE SOLUTION IS HEATED BEING SUFFICIENT TO KEEP IT LIQUID, EXTRUDING THE SOLUTION IN A CONTINUOUS UNSUPPORTED STREAM HAVING AT LEAST ONE TRANVERSE DIMENSION LESS THAN 2 MM. INTO AN EVAPORATION ZONE CONTAINING AN ATMOSPHERE HAVING A TEMPERATURE AND PRESSURE, BOTH LOWER THAN THE TEMPERATURE AND PRESSURE OF THE SOLUTION IMMEDIATELY BEFORE ITS EXTRUSION, THE PRESSURE, TEMPERATURE AND SOLVENT CONCENTRATION OF SAID ATMOSPHERE BEING EQUAL TO THOSE OF AN ATMOSPHERE IN EQUILIBRIUM WITH A SOLUTION OF THE SAME POLYMERIC SUBSTANCE IN THE SAME SOLVENT AND OF CONCENTRATION HIGHER THAN THAT OF THE SOLUTION TO BE CONCENTRATED, CONTINUOUSLY INTRODUCTING A STREAM OF GAS INTO THE SAID EVAPORATION ZONE AT A POINT REMOTE FROM THE POINT OF EXTRUSION AND CAUSING IT TO FLOW TOWARDS THE POINT OF EXTRUSION, AND IN THE COURSE OF SUCH FLOW TO MEET THE SOLUTION TRAVELLING FROM THE POINT OF EXTRUSION WHEREBY IT TAKES UP SOLVENT VAPOR FROM THE SOLUTION, AND DECREASING SAID GAS FLOW WHEN SAID VAPOR CONCENTRATION FALLS AND INCREASING SAID GAS FLOW WHEN SAID VAPOR CONCENTRATION RISES AND THEREBY MAINTAINING THE SOLVENT VAPOUR CONTENT OF THE ATMOSPHERE IN THE VICINITY OF THE POINT OF EXTRUSION ABOVE 75% BY VOLUME.