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Natural Gas Expansion and Integration with Air Separation Units Disclosure Number: IPCOM000019622D
Publication Date: 2003-Sep-23
Document File: 5 page(s) / 40K

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Natural Gas Expansion and Integration with Air Separation Units


The natural gas supply pressure from a field or treatment unit to a processing plant is usually well above the requirements of a number of unit operations within the facility. For instance, partial or catalytic oxidation (POX or CPO) of natural gas takes place at pressures under 500 psig, combustion of natural gas in gas turbines takes place at pressures under 400 psig and combustion in boiler systems occurs at near atmospheric pressure. In addition to these normal operation conditions, start-up of these facilities in remote locations often requires combustion of significant quantities of natural gas in boilers to produce steam and heat prior to internal generation of these utilities after start-up. For example, in gas-to-liquids (GTL) plants steam is raised in gas-fired boilers or electricity is generated in gas turbines to supply the start-up energy requirements of an air separation unit prior to bringing a POX unit on-line. This method of start-up results in the loss of natural gas through combustion and the need to invest in start-up boilers or turbines that may not be required during normal operation. Figure 1 depicts a typical POX-based facility for processing natural gas. An air separation unit (ASU) 9, provides industrial gases, such as oxygen for the POX unit within natural gas processing facility 4, and utilities to operate the ASU are provided by a common unit, Utilities 5. During normal operation, steam or electric power may be generated as byproducts of natural gas processing facility 4, or be generated by combustion of natural gas or off-gases. Utilities unit 5 will generally use natural gas to generate the utilities needed for start-up of air separation unit 9.

Natural Gas Expansion

A normal means of natural gas pressure reduction is by throttling across one or more control valves to the desired operating pressure(s). An alternative means is the expansion of the natural gas in a turbine or turbines loaded with a work absorbing device. The work absorbing device may be a compressor or a generator. Although turboexpansion of natural gas feed streams with subsequent compression of the processed gas or byproducts is known, the application of the available work for use in an air separation unit, and in particular to reduce the costs associated with the start-up of these units, has not been recognized.

Expansion Work Recovery

Figure 2 depicts the concept of natural gas feed pressure reduction through Expander 1 prior to entering natural gas processing facility 4. Expander 1 is loaded with Compressor 2, which compresses ambient air to be used in ASU 9. Figure 2 may consist of multiple, equal or unequal numbers of the unit operations depicted. In particular, Expander 1 may consist of one or more units feeding individual natural gas processing facilities 4, or may consist of one or more units with differing outlet pressures to feed different units with natural g...