Turboexpanders

LPG is recovered from natural gas principally by one of four extraction methods turboexpander, absorption (qv), compression, and adsorption (qv). Selection of the process is dependent on the gas composition and the degree of recovery of ethane and LPG, particularly from large volumes of lean natural gas. 

The two portions of the feed stream recombine and flow into the high pressure separator where the Hquid is separated from the vapor and is fed into an intermediate section of the demethanizer with Hquid level control. The decrease in pressure across the level-control valve causes some of the Hquid to flash which results in a decrease in the stream temperature. The pressure of the vapor stream is decreased by the way of a turboexpander to recover... 

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%. 

Adsorption. Adsorption processes have been used to recover hydrocarbons that are heavier than ethane from natural gas. Although the adsorption process has appHcations for the recovery of pentane and heavier hydrocarbons from lean gas, the percentage recovery of LPG components in these plants usually is low compared to the normal recovery of LPG in modem turboexpander or oil-absorption plants. 

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

Jotile-Thomson Valves The principal function of a J-T valve is to obtain isenthalpic coohng of the gas flowing through the valve. These valves generally are needle-type valves modified for cryogenic operation. They are an important component in most refrigeration systems, particularly in the last stage of the liquefac tion process. Joule-Thomson valves also offer an attractive alternative to turboexpanders for small-scale gas-recovery applications. 

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... 

By common usage, the terms turboexpanders and expansion turbines specifically exclude steam turbines and combustion gas turbines, which are covered elsewhere in Sec. 29. 

Any work developed by the turboexpander is at the expense of the enthalpy of the process stream, and the latter is correspondingly cooleci. A low inlet temperature means a correspondingly lower outlet temperature, and the lower the temperature range, the more effective the expansion process becomes. 

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-... 

An example of a typical turboexpander is shown in Fig. 29-46. Radial-flow turbines are normally single-stage and have combination impulse-reaction blades, and the rotor resembles a centrifugal-pump impeller. The gas is jetted tangentially into the outer periphery of the rotor and flows radially inward to the eye, from which the gas is jetted backward by the angle of the rotor blades so that it leaves the rotor without spin and flows axially away. 

The radial reaction design has been selected for turboexpanders primarily because it attains the highest efficiency of all turbine designs. However, it has several addition features which favor this apphcation ... 

Efficiency for a turboexpander is calculated on the basis of isentropic rather than polytropic expansion even though its efficiency is not 100 percent. This is done because the losses are largely introduced at the discharge of the machine in the form of seal leakages and disk friction which heats the gas leaking past the seals and in exducer losses. (The exducer acts to convert the axial-velocity energy from the rotor to pressure energy.)... 

Thrust Bearings Turboexpanders often have process upsets or ice plugging or the hke, which can cause serious thrust-bearing load variations. In applications above 506.6 to 1013.2 kPa (75 -148 psi), the best available thrust bearing usually is insufficient to protect against such high thrust loads. Various indicatious, such as the differential... 

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. 

Size, rotating speed, and efficiency correlate well with the available isentropic head, the volumetric flow at discharge, and the expansion ratio across the turboexpander. The head and the volumetric flow and rotating speed are correlated by the specific speed. Figure 29-49 shows the efficiency at various specific speeds for various sizes of rotor. This figure presumes the expansion ratio to be less than 4 1. Above 4 1, certain supersonic losses come into the picture and there is an additional correction on efficiency, as shown in Fig. 29-50. 

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... 

Figure 1-1. Modern turboexpander installation. (Source Atlas Copco.)...

In each case, the primary objective of turboexpanders is to conserve energy. Contemporary turboexpanders do this either by recovering energy from cold gas (cryogenic type) or from hot gases at temperatures of over 1,000 degrees. Current commercial models exist in the power range of 75 kW to 25-i- MW, so many applications are possible. 

Centrifugal (radial inflow) turboexpanders are well adapted to such energy conservation schemes and, with recent developments that have increased their reliability, are suitable for unattended service on a 24-hour, 7-day week operational basis. Some of the recent developments include better shaft seals, thrust bearing monitoring, and superior control devices. 

In the past, the use of the turboexpander as an energy recovery device was limited for a number of reasons ... 

What follows is a summary of turboexpander applications, an overview of what constitutes the present state-of-the-art, and the features incorporated in turboexpander design, which enable it to meet a host of power recovery requirements. 

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. 

There are numerous, large turboexpanders operating in the pressure range of 130-200 bar, most of them in well-head natural gas serviee. 

---