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Air Separation Unit and Gas-To-Liquids Facility Energy Optimization Disclosure Number: IPCOM000100152D
Publication Date: 2005-Mar-15
Document File: 2 page(s) / 41K

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Air Separation Unit and Gas-To-Liquids Facility Energy Optimization

Gas-To-Liquids (GTL) facilities convert natural gas to transportable, higher value hydrocarbons.  Successful, commercial applications are based on the partial oxidation of natural gas with oxygen to form a synthesis gas (syngas), which is upgraded to higher molecular weight hydrocarbons using Fischer-Tropsch chemistry and followed in some cases by hydroisomerization and other downstream processing.  The source of oxygen for the partial oxidation step can be air or a purified oxygen feed supplied from an air separation unit (ASU).  The ASU can be based on any or a combination of the known separation processes:  adsorption, chemical, cryogenic, polymeric membrane or an ionic transport membrane (ITM).  Commercial GTL projects have typically relied on cryogenic-based ASUs to supply oxygen.  Compared to air, purified oxygen offers many benefits for GTL processes.  However, air separation processes require energy to operate air and/or product compressors.  Typically, high pressure steam byproduct from the GTL process is used to provide the energy to generate oxygen.  A potential energy improvement is the use of internally generated process heat to improve the efficiency of the partial oxidation step.  Variously known as gas heated reforming or enhanced heat transfer reactors (EHTR), this approach lowers the oxygen requirement and changes the amounts and quality of byproduct energy available to operate air separation processes.  A second potential energy improvement is to incorporate a gas turbine into the GTL flowsheet, fueled with off-gases from the various processes.  Optimizing the energy balance in GTL facilities is a key to economic success.  Energy optimization will be an on-going focus as the efficiency of GTL processes increases with maturation of the industry.

GTL facilities have traditionally used low pressure ASUs to supply oxygen for syngas generation and small amounts of nitrogen for plant purging.  In a GTL process, mechanical work will be available from sources of byproduct steam and from the gas turbine that is used to combust byproduct off-gas.  In addition to off-gas, it may be necessary to combust additional natural gas i...