Turbo-Expander Applications in Natural Gas Processing
Authors: Maddox R.N. , Bretz K.E. 
Published: December 2007

Turbo-expander processing is the better method for efficiency of liquids recovery in this comparison of choke and expander processing of a typical natural gas stream. The advantages of low-temperature, low-pressure separation of gas liquids are presented, along with a definition of situations in which only a combination of low temperature and low pressure can be applied.

Introduction
Use of lower temperatures to increase recovery of liquid hydrocarbons from natural gas is a well known and widely practiced technique. In recent years lower and lower temperatures have been used to extract higher and higher percentages of the ethane and propane in natural gas streams. Decreased temperatures give a greater separation between those components desired in the liquid and those that would be preferred in the gas.
There are two basic ways by which the temperature of a natural gas stream can be lowered: external refrigeration and use of the internal characteristics of the gas. External refrigeration is common; in LPG recovery operations external refrigeration frequently involves a propane refrigeration system to lower the temperature propane refrigeration system to lower the temperature of the gas down to the 0 to - 40 degrees F range. Of course, external refrigeration can be used to much lower temperature levels. The cascade and mixed-refrigerant LNG cycles utilize external-refrigeration principles to achieve temperatures as low as - 260 degrees F.
Refrigeration by use of internal characteristics of the gas can be accomplished in two ways. Both ways involve the use of a pressure drop and heat exchange to obtain temperatures much lower than that of the inlet gas. The use of a choke or throttling calorimeter expansion has been common for many years. It first appeared in the early 1950's on recovery units for high-pressure gas condensate systems. The choke is simply a control valve that takes a pressure drop ranging from a few pounds to several hundred pounds. In the process of expanding across the valve the temperature of the gas is lowered.
The so-called expander-cycle processes use a "reverse-running" centrifugal compressor. In the process of expansion through the turbine the gas works on the wheel of the turbine. This work is utilized elsewhere in the process. It can be used for recompression or in cases where recompression is not necessary, or to drive electrical generators, etc.

Turbo-Expander Applications
There are, however, applications in which only expansion processes can be utilized. The characteristics of the sales gas from high-ethane recovery processes are such that processing must be carried out within fixed temperature-pressure parameters. Fig. 1 shows the pressure-temperature diagram for a typical natural-gas-sales pressure-temperature diagram for a typical natural-gas-sales gas stream after removal of 80 percent of the ethane (plus the propane and heavier constituents). Line A-C in Fig. 1 represents the dew-point line and Line B-C represents the bubble-point line for the fixed composition. Curve A-C-B represents the locus of temperatures and pressures for which a gas of this composition can pressures for which a gas of this composition can be expected to be in equilibrium with a liquid. For this reason, if the gas for processing were available at 1,000 lb, some pressure reduction would have to occur before the gas could be in the two-phase region and, therefore, subject to removal of 80 percent of the ethane by absorption or phase separation.
JPT
P. 611