Turboexpander

Type of turbine for high pressure gas

A turboexpander, also referred to as a turbo expander or an expansion turbine, is a centrifugal or axial flow turbine, through which a high pressure gas is expanded to produce work that is often used to drive a compressor or generator. Because work is extracted from the expanding high pressure gas, the expansion is approximated by an isentropic process (i. e., a constant entropy process), and the low pressure exhaust gas from the turbine is at a very low temperature, −150 °C or less, depending upon the operating pressure and gas properties. Partial liquefaction of the expanded gas is not uncommon. Turboexpanders are widely used as sources of refrigeration in industrial processes such as the extraction of ethane and natural gas liquids (NGLs) from natural gas, the liquefaction of gases (such as oxygen, nitrogen, helium, argon and krypton) and other low temperature processes. Turboexpanders currently in operation range in size from about 750 W to about 7.5 MW (1 hp to about 10,000 hp).

Although turboexpanders are commonly used in low temperature processes, they are used in many other applications. This section discusses one of the low temperature processes, as well as some of the other applications.

A refrigeration system utilizes a compressor, a turboexpander and an electric motor. Depending on the operating conditions, the turboexpander reduces the load on the electric motor by 6–15% compared to a conventional vapor compression refrigeration system that uses a throttling expansion valve rather than a turboexpander. Basically, this can be seen as a form of turbo compounding. The system employs a high pressure refrigerant (i. e., one with a low normal boiling point) such as: This is required to prevent erosion damage to the turboexpander. As shown in the figure, expansion of the flue gas through a turboexpander provides sufficient power to drive the regenerator's combustion air compressor. The electrical motor generator in the power recovery system can consume or produce electrical power. If the expansion of the flue gas does not provide enough power to drive the air compressor, the electric motor generator provides the needed additional power. If the flue gas expansion provides more power than needed to drive the air compressor, then the electric motor generator converts the excess power into electric power and exports it to the refinery's electrical system. The steam turbine is used to drive the regenerator's combustion air compressor during start ups of the fluid catalytic cracker until there is sufficient combustion flue gas to take over that task. The expanded flue gas is then routed through a steam generating boiler (referred to as a CO boiler), where the carbon monoxide in the flue gas is burned as fuel to provide steam for use in the refinery. In 1939, the Russian physicist Pyotr Kapitsa perfected the design of centrifugal turboexpanders. His first practical prototype was made of Monel metal, had an outside diameter of only 8 cm (3.1 in), operated at 40,000 revolutions per minute and expanded 1,000 cubic metres of air per hour. It used a water pump as a brake and had an efficiency of 79–83%. In 1983, San Diego Gas and Electric was among the first to install a turboexpander in a natural gas letdown station for energy recovery.

Turboexpanders can be classified by loading device or bearings. Three main loading devices used in turboexpanders are centrifugal compressors, electrical generators or hydraulic brakes. With centrifugal compressors and electrical generators the shaft power from the turboexpander is recouped either to recompress the process gas or to generate electrical energy, lowering utility bills. Hydraulic brakes are used when the turboexpander is very small and harvesting the shaft power is not economically justifiable. Bearings used are either oil bearings or magnetic bearings.