Work from steam

Useful work from steam pressure reduction... 

Steam turbines, which generate more than 80 percent of the world s electric power, differ from steam engines m that steam drives blades and not pistons. Steam turbines expand pressurized steam through nozzles that accelerate the steam at the expense of heat energy and pressure. Work is created by transferring a portion of steam velocity to blades, buckets, or nozzles affixed to a rotor to move at high speeds. Steam turbines are relatively compact in relation to steam... 

H = Total heat in 1 kg of steam at working pressure above 0°C taken from steam tables in kJ/kg. 

Distilled water is produced from sea water by evaporation in a single-effect evaporator working on the vapour compression system. The vapour produced is compressed by a mechanical compressor of 50 per cent efficiency, and then returned to the calandria of the evaporator. Extra steam, dry and saturated at 650 kN/m2, is bled into the steam space through a throttling valve. The distilled water is withdrawn as condensate from the steam space. 50 per cent of the sea water is evaporated in the plant. The energy supplied in addition to that necessary to compress the vapour may be assumed to appear as superheat in the vapour. Calculate the quantity of extra steam required in kg/s. The production rate of distillate is 0.125 kg/s, the pressure in the vapour space is 101.3 kN/m2, the temperature difference from steam to liquor is 8 deg K, the boiling-point rise of sea water is 1.1 deg K and the specific heat capacity of sea water is 4.18 kJ/kgK. 

During the generation the fluid charge in the generator is kept stirred by means of the stirring mechanism worked from G. The hydrogen produced passes through the tube condenser (where it is cooled and thus freed from steam) and then on to the gas holder. 

Chlorine dioxide degrades rapidly in air (see Seetion 6.3.2.1) and should be measurable only near its source of production or use (e.g., pulp and paper mill plants, water treatment facilities). As part of an international study of workers in the pulp and paper industry, the concentration of chlorine dioxide was measured in the workplace air of pulp and paper mills from 19 countries. The concentration of chlorine dioxide was measured in the following work areas steam and power generation (range, <0.001-0.06 ppm) effluent water treatment (range, not detected to 0.003 ppm) and maintenance (range, <detection limit to 5.8 ppb) (Kauppinen et al. 1997 Teschke et al. 1999). In another study, the concentration of chlorine dioxide was measured in the workplace air at a pulp mill in British Columbia, Canada between May and June, 1988. The concentration of chlorine dioxide was <0.01 ppm in area samples and personal full-shift samples. The exception was in the bleach/chemical preparation area sample in which the concentration of chlorine dioxide ranged from <0.01 to 0.3 ppm (Kennedy et al. 1991). 

Although noncyclic variations are possible, we shall consider a generic heat engine to be such a device, operating in a cycle, that produces useful work from a quantity of heat input in each cycle. (In principle, the spent steam could be recovered from the exhaust pipe and recycled to the boiler, so no net input is required except boiler heat.)... 

Lowering the exhaust-steam pressure always allows us to extract more work from each pound of steam. That is why we often exhaust steam to a condenser. But other than minimizing the exhaust-steam pressure, how else may we increase the amount of work that can be extracted from each pound of steam ... 

It is the velocity of the steam, impacting on the turbine wheel buckets, that causes the turbine to spin. If that is so, then the way to extract more work from each pound of steam is to increase the velocity of the steam as it escapes from the steam nozzle, shown in Fig. 17.1. 

Most of the turbines you will encounter in your work are called topping, or extraction, turbines. The idea of such a turbine is to extract much of the potential work from the motive steam, and then use the exhaust steam to reboil towers. Typically, the energy content of the exhaust steam is only 10 to 20 percent less than that of the motive steam. That is the calculation we just did with the Mollier diagram. The rest of the energy of the steam may then be used as the steam condensers, to reboil towers. This sounds pretty efficient. It is the basis for the new cogeneration projects you may have heard about. Of course, this system was used by the British Navy in the nineteenth century. 

The classical approach to the second law is based on a macroscopic viewpoint of properties independent of any knowledge of the structure of matter or behavior of mblecules. It arose from study of the heat engine, a device or machine that produces work from heat in a cyclic process. An example is a steam power plant in which the working fluid (steam) periodically returns to its original state. In such a power plant the cycle (in simple form) consists of the following steps ... 

Energy is transferred as shaft work from the steam to the surroundings by a device such as a turbine. 

A chemical plant has saturated steam available at 2,700 kPa, but bejcause of a process < has little use for steam at this pressure. Rather, steam at 1,000 kPa is liequired. Also availa saturated exhaust steam at 275 kPa. The suggestion is that the 275-kPa steam be compresse 1,000 kPa, obtaining the necessary work from expansion of the 2,700-kPa steam to 1,000 kPa. two streams at 1,000 kPa would then be mixed. Determine the rates at which steam at each i pressure must be supplied to provide enough steam at 1,000 kPa so that upon condensatio saturated liquid heat in the amount of 300 kJ s-1 is released,... 

In a long note describing his experiments on the variation of the latent heat of steam with temperature, Watt acknowledged that Mr. Southern is inclined to conclude, from the experiments on the latent heat of steam at high temperature [presented in the Appendix]... that the latent heat is a constant quantity, instead of the sum of the latent and sensible heats being so .55 This seems tantamount to an abandonment of what is known as Watt s Taw - that the sum of the latent and sensible heats is a constant. Watt had used this idea not only in his development of expansive working of steam engines but also, as we will see, it was important to his ideas about the chemical transformation of water into air.56 So, to abandon Watt s Law was to jettison a key part of Watt s chemical material theory of heat. We will see shortly, however, that a place was retained for it in a clever fashion. 

On the workings of the Newcomen Engine see R. L. Hills, Power from Steam A History of the Stationary Steam Engine (1989 Cambridge Cambridge University Press, 1993), pp. 20-30. 

We can summarize the HDA example as follows. The process converts 132 kmobh of toluene. If it were possible to extract all the work contained in the reactants we could generate 1.58 MWT of electric power at standard conditions. When forced to generate power from steam we would obtain much less than 0.65 MW of electric power. When none of the reaction energy is converted to work we need to dissipate about 1.55 MW of heat to utilities. On the separation side we need 0.52 MW of mechanical power at standard conditions. If ideal heat-driven separation devices were... 

After separation of the mixed olefins the product work up is similar to that in a steam cracker using LPG feedstock. Small amounts of carbon dioxide are removed and the hydrocarbon gases are dried before passing to a de-ethaniser column. The C2- fraction is passed to an acetylene removal unit before methane is removed from the C2 stream. This comprises 98-i-% ethylene, the remainder being ethane. The C3+ stream is split between the C3 fraction (98% propylene) and C4+. The work up of the C4 stream to produce linear butenes (not shown in the figure) is likely to be less problematic than the corresponding C4 stream from steam crackers, which is highly complex and cannot be separated by fractionation alone. The process produces little product above C5. 

It wasn t really a ship, this crudely shaped hull resting on blocks in a dusty laboratory storeroom at the University of Mercantile Marine in Kobe, Japan. Just a 12-foot, wooden-hulled model that looked as if it had been knocked together out of spare parts by a steam-fitter working from blueprints for an antiquated carnival ride. 

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