(71b) Drying Aqueous Biphenol Salt for Polyetherimide Monomer Synthesis

Polyetherimide synthesis via displacement polymerization is important for process simplification and product flexibility.  A key step in the synthesis of biphenol-based polyetherimides is the production of biphenol (BP) disodium salt with minimal moisture content.  To achieve sufficient control over the salt composition, the BP disodium salt is synthesized as an aqueous solution.  Salt drying is accomplished by introducing an aqueous salt solution into a vessel containing a hot organic solvent.  This removes most of the water from the aqueous BP salt solution as an overhead product.  The process of adding the aqueous BP salt solution into hot solvent is called “sprayover”.

During the laboratory and semi-pilot development phases of this program, we found that the sprayover vessel must be maintained at or above a certain minimum target temperature in order to avoid agglomeration of the BP salt and caking on the vessel walls.  We utilized the sprayover vessel target temperature at the 50-liter and 50-gallon scales.  However, as aqueous BP salt is sprayed into the hot solvent a large amount of solvent is lost to the overhead stream.  Such a high solvent evaporation rate causes serious solvent inventory control problems for a sprayover vessel with a short-path (non-reflux) condenser configuration.

By utilizing a partial reflux condenser in the overheads system, we avoided agglomeration and caking in the sprayover vessel and achieved stable operation at the target temperature.  We were initially concerned that the refluxing solvent, which is saturated with water, would yield a higher water concentration in the sprayover vessel (compared with the short-path condenser configuration).  However, application of the phase rule and ASPEN Plus® simulations of the solvent-H₂O system showed that the equilibrium water concentration in the sprayover vessel depends only on the sprayover vessel temperature and pressure, and is independent of the condenser configuration.  Measured solvent moisture levels were comparable for both condenser configurations.

 The calculated heat duty to operate a sprayover vessel at the target temperature is 6% higher with a partial reflux condenser than with a short-path condenser.  However, the calculated amount of solvent in the distillate at the target temperature is 33X lower with a partial reflux condenser than with a short-path condenser.

 Sprayover tests in 50-liter and 50-gallon sprayover vessels demonstrated that with a partial reflux condenser, the solvent can be contained in the system at high target temperatures.  In these tests, solvent losses were minimal, the vessel walls were clean after sprayover at both scales, and the salt product polymerized normally.