Power recovery

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. 

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The rich oil from the absorber is expanded through a hydrauHc turbiae for power recovery. The fluid from the turbiae is flashed ia the rich-oil flash tank to 2.1 MPa (300 psi) and —32°C. The flash vapor is compressed until it equals the inlet pressure before it is recycled to the inlet. The oil phase from the flash passes through another heat exchanger and to the rich-oil deethanizer. The ethane-rich overhead gas produced from the deethanizer is compressed and used for produciag petrochemicals or is added to the residue-gas stream. 

Is there any pressure letdown without power recovery ... 

In the modern unit design, the main vessel elevations and catalyst transfer lines are typically set to achieve optimum pressure differentials because the process favors high regenerator pressure, to enhance power recovery from the flue gas and coke-burning kinetics, and low reactor pressure to enhance product yields and selectivities. 

D. H. Linden, "Catalyst Deposition in FCCU Power Recovery Systems," presented at Katalistiks 7thA.nnualFCC Symposium, Venice, Italy, May 1986. 

A. P. Kreuding, "Power Recovery Techniques as AppHed to Fluid Catalytic Cracking Unit Regenerator Flue Gas," presented at 79thFEChE... 

W. C. Meyers and L. M. Stettenbenz, "The Power Recovery Gas Expander," paper 64-PET, presented at Petroleum Mechanical Engineering Conference of AS ME, Sept. 1964. 

L. A. Hissink and A. W. Drake, "Power Recovery Expander Experience," presented at Katalistiks 5th MnnualFCC Symposium, Vieima, Austria, May 1984. 

Fig. 1. Schematic of nitric acid from ammonia showing integration of reactor heat recovery, power recovery from tailgas, and air compression (3).

Power Recovery in Other Systems. Steam is by far the biggest opportunity for power recovery from pressure letdown, but others such as tailgas expanders in nitric acid plants (Fig. 1) and on catalytic crackers, also exist. An example of power recovery in Hquid systems, is the letdown of the high pressure, rich absorbent used for H2S/CO2 removal in NH plants. Letdown can occur in a turbine directiy coupled to the pump used to boost the lean absorbent back to the absorber pressure. 

Both air and oxygen processes can be designed to be comparable in the following areas product quaUty, process flexibiUty for operation at reduced rates, and on-stream rehabiUty (97,182). For both processes, an on-stream value of 8000 h/yr is typical (196). The rehabiUty of the oxygen-based system is closely linked to the rehabiUty of the air-separation plant, and in the air process, operation of the multistage air compressor and power recovery from the vent gas is cmcial (97). 

Radial-flow turbines have been developed primarily for the production of low temperatures, but they also may be used as power-recovery devices. 

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. 

In applying power recovery, three basic problems are (1) hmitations in designing equipment to recover the power, (2) operating reluctance to consider rotating equipment that is not absolutely necessaiy, and (3) the way in which the economics of the installed system is evaluated. It is important to recognize that there has always been an opera-... 

Basic to establishing whether power recovery is even feasible, let alone economical, are considerations of the flowing-fluid capacity available, the differential pressure available for the power recovery, and corrosive or erosive properties of the fluid stream. A further important consideration in feasibihty and economics is the probable physical location, with respect to each other, of fluid source, power-production point, and final fluid destination. In general, the tendency has been to locate the power-recoveiy driver and its driven unit where dictated by the driven-unit requirement and pipe the power-recoveiy fluid to and away from the driver. While early installations were in noncorrosive, nonerosive services such as rich-hydrocarbon absorption oil, the trend has been to put units into mildly severe seiwices such as amine plants, hot-carbonate units, and hydrocracker letdown. 

Economics Power-recoveiy units have no operating costs in essence, the energy is available free. Furthermore, there is no incremental capital cost for energy supply. Incremental installed energy-system costs for a steam-turbine driver and supply system amount to about 800 per kilowatt, and the incremental cost of an electric-motor driver plus supply system is about 80 per kilowatt. By contrast, even the highest-inlet-pressure, largest-flow power-recoveiy machines will seldom have an equipment cost of more than 140 per kilowatt, and costs frequently are as low as 64 per kilowatt. However, at bare driver costs (not including power supply) of 64 to 140 per kilowatt for the power-recovery driver versus about 30 to 80 per Idlowatt for... 

Development The following discussion relates specifically to the use of what could be called radial-inflow, centrifugal-pump power-recovery turbines. It does not apply to the type of unit nurtured by the hydroelecti ic industry for the 1 ge-horsepower, large-flow, low- to medium-pressure differential area of hydraulic water turbines of the Felton or Francis runner type. There seems to have been little direct transfer of design concepts between these two fields the major manufacturers in the hydroelectric field have thus far made no effort to sell to the process industries, and the physical arrangement of their units, developed from the requirements of the hydroelectric field, is not suitable to most process-plant applications. 

FIG. 29-52 Generalized curves showing hydraulic behavior of centrifugal pumps operating as power-recovery turbines. 

Performance of the power-recovery unit operating with a makeup driver (Fig. 29-56) is shown in Fig. 29-57 specific percentage values are shown, but the general characteristics and curwe shapes are typical. It should be noted that the flow scheme, the selection of... 

Example 2 Units for a Power Recovery System The scheme of Fig, 29-52 and the actual units supplied (Figs, 29-58 and 29-59) will he used for purposes of illustration. Since this case has the recovery unit as the sole driver, the balance point is set hy speed and horsepower. 

FIG. 29-55 Performance of a power-recovery unit operating as the sole driver. 

FIG. 29-56 Flow diagram of a system with a power-recovery turbine operating with a makeup driver. 

FIG. 29-58 Head-horsepower-capacity characteristics of a lean pump tandem-connected with a power-recovery turbine operating as the sole driver. To convert gallons per minute to cubic meters per minute, multiply by 3.79 X 10 to convert horsepower to kilowatts, multiply by 0.746 and to convert pounds-force per square inch to megapascals, multiply by 6.89 X 10 . ... 

FIG. 29-59 Head-horsepower-capacity characteristics of a power-recovery turbine operating as the sole driver of a lean pump. If the total capacity of lean and semilean pumps exceeds the values indicated by available head limit, bypass must be used. Net recovery-pump head at 8.71 mVmin (2300 gal/min) is figured as follows ... 

The return on capital investment did not justify a power recovery system unless more tlian several tliousand horsepower was recovered. 

Lack of confidence in new power recovery schemes that were not yet proven made both government and private industry reluctant to invest in these systems. 

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 cycles in diese power recovery applications are relatively simple. They involve the removal of solids or liquids ahead of the expander, and often the incoming stream is heated so its temperature will not reach its frost point at the expander discharge. This heating also increases the amount of available power. Some examples of this... 

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