Expanders, turboexpanders

Valves are often used to reduce the pressure of a gas or liquid process stream. By replacing the valve with a turbine, called an expander, turboexpander, or expansion turbine in the case of a gas and a liquid expander or radial-infiow, power-recovery turbine in the case of a liquid, power can be recovered for use elsewhere. Power recovery from gases is far more common than from liquids because for a given change in pressure and mass flow rate, far more power can be recovered from a gas than from a liquid because of the lower density of the gas. Equations for f.o.b. purchase costs of power recovery devices are included in Table 16.32 in terms of horsepower that can be extracted. Typical efficiencies are 75-85% for gases and 50-60% for liquids. Condensation of gases in expanders up to 20% can be tolerated, but vapor evolution from liquid expansion requires a special design. Whenever more than 100 Hp for a gas and more than 150 Hp for a liquid can be extracted, a power recovery device should be considered. 

Essentially all of the methane [74-82-8] is removed ia the demethanizer overhead gas product. High recovery of ethane and heavier components as demethanizer bottoms products is commonplace. The work that is generated by expanding the gas ia the turboexpander is utilized to compress the residue gas from the demethanizer after it is warmed by heat exchange with the inlet gas. Recompression and deUvery to a natural gas pipeline is performed downstream of the plant. A propane recovery of 99% can be expected when ethane recoveries are ia excess of 65%. 

Turboexpanders can be classified as either axial or radial. Axial flow expanders have either impulse or reaction type blades and are suitable... 

The need to obtain greater recoveries of the C9, C3, and C4S in natural gas has resulted in the expanded use of low-temperature processing of these streams. The majority of the natural gas processing at low temperatures to recover light hydrocarbons is now accomphshed using the turboexpander cycle. Feed gas is normally available from 1 to 10 MPa. The gas is first dehydrated to a dew point of 200 K and lower. After dehydration the feed is cooled with cold residue gas. Liquid produced at this point is separated before entering the expander and sent to the condensate stabilizer. The gas from the separator is... 

The turboexpander in combination with a compressor and a heat exchanger functions as a heat pump and is analyzed as follows In Fig. 29-44 consider the compressor and aftercooler as an isothermal compressor operating at To with an efficiency and assume the working fluid to be a perfect gas. Further, consider the removal of a quantity of heat by the tumoexpander at an average low temperature Ti-This requires that it dehver shaft work equal to Q. Now, make the reasonable assumption that one-tenth of the temperature drop in the expander is used for the temperature difference in the heat exchanger. If the expander efficiency is and this efficiency is mul-... 

If hquid droplets form as ihe gas is expanded in the turboexpander, one s first thought may be that a radial inflow design is the last diing to use, but the following explanation will show that this is the only design that can accomphsh expansion efficiently. 

Turboexpanders are expansion turbines, rotating maehines similar to steam turbines. Commonly, the terms expansion turbines and turboexpanders speeifieally exelude steam turbines and eombustion gas turbines. Turboexpanders (Figure 1-1) ean also be eharaeterized as modern rotating deviees that eonvert the pressure energy of a gas or vapor stream into meehanieal work as the gas or vapor expands through the turbine. If ehilling the gas or vapor stream is the main... 

For many years, turboexpanders have been used in cryogenic processing plants to provide low-temperature refrigeration. Power recovery has been of secondary importance. Expander efficiency determines the amount of refrigeration produced and, in gas process plants, the amount of product usually depends on the available refrigeration. Accordingly, there is a large premium on efficiency and, of course, on reliability. 

The main market for turboexpanders has been in low-pressure air separation plants, expanding down from 5 bar, and in hydrocarbon processing plants, expanding natural gas from as high as 200 bar. The air separation expanders are roughly divided into two types. The first type ranges from a few horsepower up to 100 hp. Here, the expander power is too small to be economically recovered and is, therefore. 

Dust-laden streams can also cause operational problems. A turboexpander that can efficiently process condensing streams (gas with fog droplets suspended) can usually handle a stream with suspended solid particles, as long as the particle size does not exceed 2-3 p. The newer designs reduce erosion of expander back rotor seals by disposing of... 

The success of expanders was predicted in the 1940s. More recently, processes similar to those used in air separation have been applied in other fields. These new applications have progressed as a result of the parallel development of new processes and improved heavy-duty turboexpanders. 

A few separation plants have reeiproeating expanders for 2,000 to 3,000 psi (13,800-20,700 kN/m ) inlet pressure. The ineoming pressurized gas is about -40°F (-40°C) and is not elean enough to operate satisfaetorily in small turhoexpanders. However, several turboexpanders have been put into air serviee during the last deeades at 1,500 psia (10,300 kN/m ) for liquid produetion. 

There is a Second Law thermodynamic advantage in operating an expander at as low a temperamre as possible. In most applications it has been aiTanged to discharge just above tlie dew point of tlie expanded gas. If the cold compressed gas could enter tlie expander at or near its dew point, the expander would then operate condensing and at the lowest possible temperamre. Such condensate has traditionally been troublesome in turbines, but tliis has been solved in modern turboexpanders. 

Pressures Turboexpanders ean be designed to operate at up to 3,000 psi and higher inlet pressures as required by eonditions. Expansion pressure ratios ean also be adjusted for eaeh proeess over a wide range. A majority of effieient expansion ratios are below 5 1, although pressure ratios up to 10 1 ean be aeeommodated with reasonable effieieney. Smaller, lower pressure units are popular for air separation and helium liquefaetion. Intermediate pressure (100-1,000 psi) and high pressure expanders (1,000-3,000 psi) are widely used in natural gas proeessing and industrial gas liquefaetion. 

Gas can be condensed by (a) mechanically refrigerating it, (b) compressing and expanding it, using turboexpanders, or, (c) pressure effects such as by Joule-Thomson cooling and overcoming the vapor pressure. The liquefaction of methane can involve all three of these effects. These effects can be separately evaluated to show the effectiveness of each in producing liquid. 

Most ethylene plants operate continuously with the expanders operating at or near design conditions. If necessary, due to their unique design characteristics, radial inflow turboexpanders can accommodate a wide range of process conditions without significant losses in thermal or mechanical efficiency. Expanders may be loaded with booster compressors, gear-coupled generators, dynamometers, or other in-plant mechanical equipment such as pumps. In ethylene plants, turboexpanders are typically used in eitlier post-boost or pre-boost applications. 

In pre-boost applications, the turboexpander discharge pressure is considerably lower than the compressor inlet pressure. This situation requires special consideration when designing the turboexpander and auxiliary systems. In pre-boost designs, compressing the gas to a higher pressure produces more refrigeration in the turboexpander. Figure 3-9 shows a cross-section of an expander-compressor unit. 

The turboexpander should tolerate the proeess gas stream at a saturated state and eondensation through the expander wheel. 

Tail gas expanders are thus an integral part of modern nitrie aeid plants. However, these turboexpanders are also part of a eombined turbomaehinery train eomprised of a prime mover and two or more eompressor easings. 

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