Process for hydrogen recovery from ammonia purge gases

A process for synthesizing ammonia from hydrogen and nitrogen comprising;

• a. introducing to and reacting in an ammonia synthesis reaction zone containing an ammonia synthesis catalyst a nitrogen and hydrogen-containing reactor feed gas at a superatmospheric synthesis pressure of about 100 to 350 atmospheres absolute to produce ammonia, said reactor feed gas having a mole ratio of hydrogen to nitrogen of about 2.9:1 to 3.3:1 and comprising inert contaminants;
• b. withdrawing an ammonia-containing reaction effluent from said ammonia synthesis reaction zone and recycling said reaction effluent to said ammonia synthesis reaction zone in an ammonia synthesis loop, said reaction effluent comprising hydrogen, nitrogen, ammonia and at least one of methane and argon wherein the reaction effluent comprises about 10 to 25 volume percent ammonia;
• c. removing a purge stream from said ammonia synthesis loop in an amount sufficient to maintain the concentration of inert contaminants in the reactor feed gas less than about 25 volume percent;
• d. cooling said purge stream at a pressure of at least substantially said superatmospheric synthesis pressure to coalesce ammonia and separating the coalesced ammonia from the purge stream to provide a purge stream containing about 1 to 4 volume percent ammonia;
• e. contacting the purge stream containing about 1 to 4 volume percent ammonia at a pressure of at least substantially said superatmospheric pressure with a separation membrane, said separation membrane having a feed side and a permeate exit side and exhibiting selective permeation of each of hydrogen and ammonia as compared to the permeation of each of methane and argon;
• f. providing said permeate exit side of said separation membrane at a lower total pressure said lower pressure being at least about 20 atmospheres less than the pressure at said feed side of the membrane;
• g. permeating through and recovering from the permeate exit side of said separation membrane a permeating gas comprising at least about 20 percent of the hydrogen and at least about 20 percent of the ammonia in the purge stream;
• h. removing from the feed side of said separation membrane a non-permeating gas;
• i. recycling said permeating gas to said ammonia reaction zone;
• j. introducing a synthesis feed gas into said ammonia synthesis loop after said purge stream has been withdrawn from the ammonia synthesis loop, said synthesis feed gas being at substantially said superatmospheric synthesis pressure and comprising nitrogen, hydrogen, and at least one of methane and argon, wherein the mole ratio of hydrogen to nitrogen is less than the mole ratio of hydrogen to nitrogen in the reactor feed gas and is about 2.7:1 to 3.2:1;
• and
• k. removing ammonia from the reaction effluent in the ammonia synthesis loop after introducing the synthesis feed gas into the ammonia synthesis loop.