Removing trace amounts of water from agricultural grade anhydrous ammonia

Agricultural grade anhydrous ammonia (liquefied gas) contains 2000 ppm $\ce{H2O}$. What would be the process to remove this water to obtain refrigeration grade (<100 ppm $\ce{H2O}$) anhydrous ammonia? 




Calcium oxide can get water down to 4ppm in anhydrous ammonia according to <http://www.google.com/patents/US4075306>.


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