Turbo-expanders

Agahi, R. R. and Ershaghi, B., 1993. Engineering of Turbo-expander-Compressor Units for Off-Shore Installation, Eifth International Congress on Eluid Maehinery, The Hague, Netherlands. 

The CO2 rich solvent is drained from the bottom of the tower, and led first to a hydraulic turbo-expander and then to four flash drums connected in series, where CO2 is de-absorbed as the pressure is lowered. Lean solvent is pumped back to the top of the absorber tower... 

In some larger units, a turbo expander is used to recover this pressure energy. To protect the expander blades from being eroded by catalyst, flue gas is first sent to a third-stage separator to remove the... 

A power recovery train (Figure 1-12) employing a turbo expander usually consists of four parts the expander, a motor/generator, an air blower, and a steam turbine. The steam turbine is primarily used for staii-up and, often, to supplement the expander to generate electricity. 

Figure 8-2 The energy that may be recovered from gases by using turbo-expanders.

Fig. 5.14. Flow diagram of the Linde TCF series of turbine helium liquefiers. C denotes the screw compressor, OF the oil filter, Ej 2 3 the heat exchangers, T12 the turbo-expanders and JT the final Joule-Thomson stage. Cylinder B is for pure He gas storage and Gas He is the normal He gas input to the purifier [60],...

Turbine type impellers, 16 699 Turbo expanders, in cryogenic processes, 13 698... 

In 1996, the World Bank funded a study by John Brown Engineering, to examine the Karadagh GPP. The study recommendations included proposals for a new, two stream facility comprising modern technology, turbo expanders, and feed gas compression. 

Thus these two installations are principally different. 3S allows reaching Iowa- temperatures because early starts isentropic process and has significant pressure drop within the device. But this pressure drop can be recovered in the diffusa-. Moreova- it is insensitive to the presence of liquids (which in turbo expander can damage the turbine) and seems not to experience the hydrates formation. 

Table 7.1 lists the exergy values for CH4 ("natural gas") at various conditions. A gas reservoir stores natural gas at 300 bar while this gas after treatment is distributed at 70 bar. Which fraction of the gas total exergy is, by approximation, lost by this expansion Would this justify the installation of a turbo expander ... 

If, in the heat pumps, the energy of compression is not recovered but is wasted in letdown valves (as the pressure of the working fluid is reduced to the low pressure of the evaporator (Joule-Thomson cycle), the liquefaction efficiency will be low (35-60%). This range of efficiencies is a function of the liquefier size and refrigerant used. If the letdown valves are replaced by turbo expanders (Brayton cycle), which recover some of the compression energy during pressure letdown, and if helium or neon refrigerants are used, the efficiency can theoretically reach 80-90%. 

Refrigeration down to 80°K is therefore provided by LN2. The next step of refrigeration from 80 to 30°K is carried out using the Brayton cycle, in which high-pressure H2 is expanded in a number of turbo expanders in series. From 30°K to liquefaction, the Joule-Thomson cycle is used, where high-pressure gas is throttled to low pressure to provide further cooling. 

Trepp, C., "Refrigeration Systems for Temperatures Below 25°K with Turbo Expanders," in Advances in Cryogenic Engineering, Vol. 7, K. D. Timmerhaus, Editor, Plenum Press, New York (1961). 

Development of the turbo-expander process allowed the design and construction of plants for recovery of liquid ethane, as well as the heavier hydrocarbon components. The turboexpander extracts useful work from the gas during expansion from a high pressure to a lower pressure. Because of the work extraction... 

Design, plant testing and industrial trials of the ACEC turbo expander... 

Often the gas pressure is sufficient to use a turbo-expander which cools the gas to below 0 C and causes further condensation of hydrocarbon liquids. Because the gas stream also contains water, gas hydrate and ice formation can be a problem. This is prevented by the addition of an additive such as methanol. Following the removal of condensate, the gas stream is dried and if necessary treated further to remove acid gases such as hydrogen sulphide and carbon dioxide. 

The basic flow for a turbo-expander scheme is illustrated in Figure 3.2. This represents the simplest flow diagram, which can be quite complex if ethane is to be extracted". 

Over time turbo-expander systems have improved in efficiency and can be used to extract ethane by inclusion of gas-to-gas heat recovery systems. These are variously described as cryogenic systems or cold boxes and are similar in operation to the cryogenic units used for the production of LNG. The use of cold-boxes permits pre-cooling of the gas before the turbo-expander and hence an overall colder operation, this is illustrated in Figure 3.3. 

One advantage of the turbo-expander method for separating LPG from natural gas is that it allows the use of gas-pipelines to transport the LPG. LPG is costly to store and transport as it requires pressurised or cryogenic-vessels. By using gas pipelines, the lower cost transport economics of pipeline gas can be used. 

In the straddle plant option, LPG is left in the sales gas at the gas-plant. The much larger volume of methane dilutes the LPG and the gas including the LPG meets the pipeline dewpoint specification. The mixture is then piped over several hundred kilometres to the straddle plant. This uses a turbo-expander to separate LPG Irom the gas, maintaining the residual gas within the heating value specification. There are several such operations in Canada and Australia which have been described by Hawkins ... 

Before the advent of turbo-expander plants in the early 1970s, the preferred method for removal of LPG materials from the gas stream was by absorption in a suitable solvent. To increase the absorption efficiencies, especially for the recovery of ethane, this technology was developed by applying refrigerated solvent to the gas stream. 

The refrigerated absorber technology is complex and manpower intensive compared to the turbo-expander technology that has largely replaced it. However, where they still exist they are particularly useful for recovering ethane, which is more difficult to extract in turboexpander plants without refrigerated cold-boxes. ... 

The typical US well head cost in 2007 was around 6.37/GJ and this has been used as the basis for the input cost in this case study. The fixed costs are the non feed operating costs which for a relatively simple turbo-expander gas plant would be about 5% per annum of the fixed capital and the capital recovery charge which is placed at 14.3% per annum of the capital (see Appendix for derivation of this value). 

A significant improvement to the process outlined above was the use of an expansion engine. Typically, in ASUs turbo expanders are used. An ideal turbo expander is isentropic and reversible. Illustrated in Figure 3.9, air at -150°F (172 K) and 90 psia (620 kPa) is expanded to 20 psia (138 kPa). In an isentropic expansion A-B, the expansion follows the isentrope with a net change in enthalpy. In reality the expansion will not be reversible and will follow a curve similar to A-C. The actual enthalpy change divided by the isentropic enthalpy change is a measure of the expander efficiency. 

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