Steam, operating principles

The operating principle of steam-jet ejectors is explained in Volume 1, Chapter 8. Then-specification, sizing and operation are covered in a comprehensive series of papers by Power (1964). Diffusion pumps are used where very low pressures are required (hard vacuum) for processes such as molecular distillation. 

The high capital investment cost of the Asahi process is due to the necessity for large absorbers, evaporators, crystallizers, dryers, rotary kiln crackers and screw decanter separators. The major operating and maintenance costs are electricity, fuel oil, steam and chemicals such as soda ash, EDTA and limestone. The requirement for consumption of large amounts of utilities is associated with the operation principle and design of the Asahi process. According to the economic evaluation, equipment required for N0X and SO2 absorption (such as packed-bed absorbers) accounts for 20% of total direct capital investment for treatment of dithionate ion (such as evaporator, crystallizer, dryer, and cracker) it accounts for about 40% and for treatment of nitrogen-sulfur compounds (such as screw decanter and cracker) it accounts for only 2%. 

The mobile O2- ions in YSZ move from the cathode to the anode, the water is produced at the anode, and it can be removed together with unreacted hydrogen and C02 (in the case of hydrocarbon combustion). For internal reforming the anode reaction also provides the necessary water for steam reforming. The operation principles of a SOFC with YSZ as electrolyte is shown in Fig. 13. 

The dehydrogenation of butenes in the presence of steam was developed initially by Esso, Shell and Phillips. In accordance with the operating principles of this type of process, the preheated feed is mixed with superheated steam and then sent to adiabatic reactors containing catalyst beds 80 to 90 cm thick. The temperature, initially 620 C, must be raised progressively as catalyst activity decreases. The latter is regenerated by simple steam treatment The reaction pressure is 0.1 to 0.2.10A Pa absolute, and reaches 0.5. 1C6 Pa absolute during regeneration. 

As noted above, any direct dryer, in principle, can be converted to superheated steam operation (e.g., flash, fluid bed, spray, impinging jet, conveyor dryers, etc.). Thermal efficiencies can be improved and the unit size reduced by supplying a part of the heat indirectly (e.g., by conduction or radiation). Note that the inversion temperature is lowered in the presence of indirect heat supply, which is a further benefit. For example, in the presence of appropriate radiant heating, the inversion temperatnre dropped from 250 C to ITO C for a specific case reported in the literature. 

Balanced pressure, thermostatic operating principle = vapor pressure of fluid inside bellows. Not for superheated steam, corrosive condensate or waterhammer. 

Thermodynamic/kinetic energy intermittent, operating principle is Bernoulli s principle/impulse, poor air handling, larger sizes more susceptible to back pressure. Usually for steam pressure < 1.2 MPa but > 0.06 MPa. Affected by ambient temperature. Discharge pressure < 0.5 steam pressure. Range 0.06-0.3 kg/s. Size on 3-4 X usual flowrate of condensate. 

Fig. 24. Operating principle of a membrane reactor for steam methane reforming.

Jackson, J., Steam Boiler Operation Principles and Practices, 2nd edn, Prentice Hall, London (1987)... 

Drum Drying. The dmm or roHer dryers used for milk operate on the same principles as for other products. A thin layer or film of product is dried over an internally steam-heated dmm with steam pressures up to 620 kPa (90 psi) and 149°C. Approximately 1.2—1.3 kg of steam ate requited per kilogram of water evaporated. The dry film produced on the roHer is scraped from the surface, moved from the dryer by conveyor, and pulverized, sized, cooled, and put iato a container. 

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