Throttled expansion

The Linde liquefaction process, which depends solely on throttling expansion, is shown in Fig. 9.6. After compression, the gas is precooled to ambient temperature. It may be even further cooled by refrigeration. The lower the temperature of the gas entering the throttle valve, the greater the fraction of gas that is liquefied. For example, a refrigerant evaporating in the cooler at 233.15 K (—40°C) provides a lower temperature at the valve than if water at 294.15 K (21°C) is the cooling medium. 

Because of the variation of the Joule-Thomson coefficient with both temperature and pressure it is not easy to calculate the change of temperature resulting from a given throttled expansion, even when such data as in Table IV are available. This can be done, however, by a series of approximations. By estimating a rough average for the Joule-Thomson coefficient, some indication of the fall of temperature can be obtained. 

Problem Estimate the final temperature accompanying the throttled expansion of nitrogen, initially at 25 C, from 200 atm. to 1 atm. pressure. 

Ideal turbine (100% efficiency) (2) Turbine with 80% efficiency (3) Throttle expansion. 

In the air separation process (see Fig. 2.3A) liquid high pressure air (c), resulting from the internal compression of oxygen, is expanded via a throttle valve (22) into the pressure column (12). Alternatively this expansion can be performed in a so called dense fluid expander . This is a turbine for the expansion of a liquid or very dense supercritical cold fluid. A turbine expansion produces less exergy loss than a throttle expansion. Owing to this the use of a dense fluid expander reduces the work for gas separation or liquefaction. 

Throttle expansion down to 25 bar into a separator. The condensate in the separator has a neon content of about 97%. 

The energy consumption of the process depends quite critically on the method of product recovery. In the cases shown in Figures 1.16 to 1.18, a throttling expansion is used to lower the pressure and hence the dissolving... 

The energy requirements for dense gas extraction will depend on the method employed for solvent regeneration. With careful design they should not exceed those for a conventional solvent recovery process and they may be considerably lower. Energy consumption for processes where solvent recovery involves a throttling expansion are discussed in chapter 9. 

Steam Rate Enthalpy data can be obtained from Mollier diagrams or from steam tables (see Sec. 2), from which the theoretical steam rate can be calculated. For example, a throttle inlet condition of 4137 kPa (600 psig) and 399° C (750° F) gives an enthalpy of 3.2 MJ/kg (1380 Btu/lb), and if the end point is at 348 kPa (50 psig), then adiabatic expansion is to 2.69 MJ/kg (1157 Btu/lb). This gives 0.52 MJ/kg (223 Btu/lb) available, and the theoretical steam rate is calculated from the Btu equivalent per Idlowatthour or horsepower-hour ... 

Throttling The expansion of a fluid through a constricted passage (across which there is a pressure difference), during which no external work is done. The initial and final velocities of the fluid are equal, and there is no heat exchange with external sources. A change in entropy will, however, take place. 

Expansion turbines are related in many design features to the centrifugal compressor. The key exception being that the turbine receives a high pressure gas for expansion and power recovery to a lower pressure and is usually accompanied by the recovery of the energy from the expansion. For example, applications can be (1) air separation plants (2) natural gas expansion and liquefaction (for gas let-down in pipeline transmission to replace throttle valves where no... 

Where high-pressure gas or liquid process streams are throttled to lower pressures, energy can be recovered by carrying out the expansion in a suitable turbine. 

Source models for throttling releases require detailed information on the physical structure of the leak they are not considered here. Free expansion release source models require only the diameter of the leak. 

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