 |
|
IPSI Gas Processing Papers & Publications
Flexibility, efficiency to characterize gas-processing technologies in the next century
Published in the Oil & Gas Journal, 1999 Petroleum in the 21st Century, December 12, 1999
Authors: R. J. Lee, Jame Yao, Douglas Elliot, IPSI LLC
Technologies in gas processing that have evolved since 1980 point towards major trends in natural gas processing in the early years of the 21st Century.
These technologies have arisen to allow a much more rigorous optimization of several variables: operational flexibility, CO2 tolerance, and capital and operating cost.
The selection of an optimum process will depend on conditions and composition of the inlet gas, cost of fuel and energy, product specifications, and relative product values.
Increased attention to thermodynamic efficiency and better use of refrigeration and recompression since 1980 have led to development of enhanced NGL recovery processes that are mostly proprietary and available for license.
These processes are equally applicable to the retrofit of existing less efficient plants. In many cases, the economic return on retrofit investments is much more attractive than new plants.
What follows is a review of the major technologies that will likely be in the forefront for new and retrofitted plants in the next century.
Ethane recovery economics
Recovery of NGL components in gas not only may be required for hydrocarbon dew point control in a natural gas stream, but also yields a source of revenue. Lighter NGLs, such as C2, C3, and C4s, can be sold as fuel or feedstock to refineries and chemical plants, while the heavier portion can be used as gasoline-blending stock
The price difference between selling NGL as a liquid and as fuel, commonly referred to as the "shrinkage value," often dictates the recovery level desired by the gas processors.
Advances in gas processing technology in conjunction with improvements in equipment design and efficiency have attracted the processors to invest in gas plants with high liquid recoveries in recent years.
In addition to their simplicity and lower recompression requirements, new processes are often designed with great flexibility and easy transition between ethane rejection and ethane recovery, allowing the gas processor to respond quickly to the cyclic ethane liquid market, a phenomenon that seems to occur frequently in today’s market.
NGL recovery: technical trends
In the 1950s and 1960s, a simple oil-absorption process was commonly utilized to recover propane in the 25 to 50% ranges. This process used light oil to absorb NGL components from the feed-gas stream in an absorption column, typically operated at the ambient temperature and a pressure close to the sales-gas pipeline pressure.
The late 1980s saw introduction of a proprietary selective solvent process based on the refrigerated lean-oil-absorption process in which a lighter oil used as the physical solvent is selectively tailored for recovery of desirable hydrocarbons from a gas stream.3
This process also provides the flexibility of recovering only selective hydrocarbons economically desirable: recovering or rejecting ethane, for example.
Beginning around 1960, as the demand for ethane increased, new processes were sought to recover ethane more efficiently. For ethane recovery up to 50% and LPG recovery up to 90%, a simple propane refrigeration system provides refrigeration at temperatures to -40°F.
For higher recovery, 80% ethane recovery, for example, a cascade refrigeration cycle would be required to provide refrigeration at a lower temperature to approximately –90°F.
The use of turbo-expanders in gas-processing plants began in the early 1960s as a result of technological improvements in the manufacture of practical mechanical designs suitable for continuous operation in a variety of operating conditions.
By 1970, most new gas processing plants were being designed to incorporate the particular advantages of an expander producing useable work normally used partially to recompress residue or export gas. The gas processing industry is now dominated by turbo-expander plants for NGL recovery.
Coincident with development of reliable turbo-expanders was development of compact, efficient, and relatively inexpensive plate-fin or brazed aluminum heat exchangers.
The combination of these two pieces of equipment, together with the availability of accurate thermophysical properties and thus reliable process simulation software, has allowed process designers to achieve significant improvements in the cost and flexibility of turbo-expander plants.4