Amoco Chemical Company and now bp Amoco Chemical Company (bp) have continued to improve their xylene isomerization catalysts and pX crystallization processes since commissioning their first commercial pX unit in 1967. pX separation from isomerized reformate mixed xylenes feedstocks requires first stage crystallization section temperatures as low as -90 F (-68 deg C). The current bp processes use cascaded ethylene/propane or ethylene/propylene (C2/C3) vapor compression refrigeration systems, wherein the C2 refrigeration loop effects pX crystallization, and the C3 refrigeration system is used primarily to desuperheat and condense ethylene. The C3 compressor is an expensive equipment item, and the largest single user of electricity on the unit. This paper will describe new energy efficient pX processes that use waste heat powered ammonia absorption refrigeration (AAR) instead of conventional C3 refrigeration. The large amount of electricity used to power the C3 refrigeration compressor in conventional processes, will be replaced by waste heat used to power AAR. Waste heat is essentially free energy that is currently lost via air or water cooling. The new processes will reduce electricity consumption for a 400 KMTA pX unit based on reformate xylenes by about 5 MW, making them the lowest energy consuming pX processes available. Recent advances in AAR technology make the capital cost of AAR comparable to C3 vapor compression systems.
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