Power recovery from liquid streams

The potential for power recovery from liquid streams exists whenever a liquid flows from a high-pressure source to one of lower pressure in such a manner that throttling to dissipate pressure occurs. Such throtthng represents a system potential for power that is the reverse of a pump—in other words, a potential for power extraction. Just as in a pump, there exists a hydraulic horsepower and a brake horsepower, except that in the recoveiy they are generated or available horsepowers. 

Eric Jenett, M.S.Ch.E., Manager, Process Engineering, Brown ir Root, Inc. associate member, AIChE, Project Management Institute Registered Professional Engineer (Texas). (Power Recovery from Liquid Streams)... 

As liquids are essentially incompressible, less energy is stored in a compressed liquid than a gas. However, it is worth considering power recovery from high-pressure liquid streams (> 15 bar) as the equipment required is relatively simple and inexpensive. Centrifugal pumps are used as expanders and are often coupled directly to pumps. The design, operation and cost of energy recovery from high-pressure liquid streams is discussed by Jenett (1968), Chada (1984) and Buse (1985). 

Recovery of power from the thermal energy of a high temperature stream is the subject of Example 1.5. A closed circuit of propane is the indirect means whereby the power is recovered with an expansion turbine. Recovery of power from a high pressure gas is a fairly common operation. A classic example of power recovery from a high pressure liquid is in a plant for the absorption of CO2 by water at a pressure of about 4000 psig. After the absorp-... 

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. 

The power loss from a process liquid stream going through a pressure letdown valve could be significant. If a power expander is considered to replace the letdown valve, equation (8.1) could be used to estimate the power recovery potential, and a typical power generation efficiency for a liquid expander is 75%. 

For the process shown in Figure 4.1, a process liquid stream from the hot separator with mass rate of 1817.2gpm and pressure of 2635.3 psig reduces its pressure through a pressure letdown valve to the hot flash drum at 285.3 psig. Equation (8.1) gives an estimate of power recovery potential of 1.38 MW if a liquid expander is installed. 

Dust Filter. The cloth or bag dust filter is the oldest and often the most reliable of the many methods for removing dusts from an air stream. Among their advantages are high (often 99+%) collection efficiency, moderate pressure drop and power consumption, recovery of the dust in a dry and often reusable form, and no water to saturate the exhaust gases as when a wet scrubber is used. There are also numerous disadvantages maintenance for bag replacement can be expensive as well as a sometimes unpleasant task these filters are suitable only for low to moderate temperature use they cannot be used where liquid condensation may occur they may be hazardous with combustible and explosive dusts and they are bulky, requiring considerable installation space. 

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