High capacity purification of thermally unstable compounds

Abstract

Separations processes and apparati capable of purifying thermally sensitive materials at high capacity. An apparatus having a rectification section and a stripping section with the stripping section having a stripping tray with 5 to 50% open area, a pressure drop from 0.02 psi to 0.2 psi and a tray efficiency during operation of the column which is equal to or greater than 20%.

We claim:  
     
         1 . A separations apparatus comprising: 
 a rectification section;    a stripping section having at least one stripping tray;    said at least one stripping tray extends across the cross-sectional area of the rectification or stripping section and has a tray cross-sectional area and a plurality of open areas through which vapor may pass, wherein each of the open areas has an open cross-sectional area, wherein the sum of all of said open cross-sectional areas has a value in the range of from about 5% to about 50% of the tray cross-sectional area, and wherein    said at least one stripping tray produces a tray pressure drop during operation of the apparatus and said tray pressure drop is in a range of from about 1 mm Hg (0.02 psi) to about 10 mm Hg (0.2 psi), and    said at least one stripping tray has a tray efficiency during operation of the column which is equal to or greater than 20%.    
     
     
         2 . The apparatus of  claim 1 , wherein said stripping section has a first stripping tray and a second stripping tray which are separated by a vertical distance that is at least 8 inches.  
     
     
         3 . The apparatus of  claim 1  wherein said at least one stripping tray does not have a downcomer.  
     
     
         4 . A method of purifying acrylic acid utilizing the apparatus of  claim 1  comprising the step of producing a bottoms stream having acrylic acid in a concentration of greater than 50 wt %.  
     
     
         5 . A method of retrofitting an existing distillation column to achieve the apparatus of  claim 1  comprising the step of replacing at least two trays in the stripping section with a single tray as defined in  claim 1  capable of producing a tray pressure drop during the operation of the apparatus in the range of about 4 mmHg (0.08 psi) to about 6 mmHg (0.12 psi).  
     
     
         6 . The apparatus of  claim 1  comprising: 
 a distillation column having a feed inlet, an overhead outlet, a bottoms outlet, a reflux inlet and a reboiler return inlet, said column also having a column internal cross-sectional area bounded by an internal column diameter; wherein  
 said distillation column houses said rectification section and said stripping section,  
 said rectification section being located above said feed inlet and said stripping section located below said feed inlet.  
 
     
     
         7 . The apparatus of  claim 6  having a distillation column wall with an internal surface defined by the internal column diameter wherein said open areas allow vapor to flow through at a sufficient velocity to entrain a liquid present on an upper surface of the at least one stripping tray providing liquid mass transfer such that said liquid contacts said internal surface so as to wet the internal surface.  
     
     
         8 . The apparatus of  claim 6  further comprising, a plurality of trays sufficient to provide 8 or more theoretical stages of separation, wherein said rectification section has sufficient rectification trays to provide at least 4 theoretical stages of separation and said stripping section has sufficient trays to provide at least 4 theoretical stages of separation.  
     
     
         9 . A method of purifying thermally unstable compounds comprising: 
 employing the apparatus of  claim 6 .    
     
     
         10 . The method of  claim 9  comprising the step of producing an overhead stream having an acrylic acid composition of less than 10 wt %.