Plant preheating

When easy and logical procedures are defined there will be no need for special attention or parallel activities during the "critical" phase of production start-up. Therefore some actions are taken prematurely, in an order not exactly required at that time of the start-up procedures (example caustic scrubber recycle and caustic feed, not necessary during plant preheating, will be started in advance to accumulate sufficient excess of caustic to absorb the initial extra amount of SO2 produced during the initial start-up period after sulphur ignition). 

Figure 10.5. Ammonia synthesis loop. This layout is typical of a loop for a large capacity (1000 tonne day ) plant. Preheat of the converter feed gas to 150 C allows high-grade heat recovery from the converter exit gas. Reprinted from Catalyst Handbook, 2 ed., Ed. by M. Twigg, Copyright (1989) by kind permission of M. Twigg.

Ma.nufa.cture. Butyrolactone is manufactured by dehydrogenation of butanediol. The butyrolactone plant and process in Germany, as described after World War II (179), approximates the processes presendy used. The dehydrogenation was carried out with preheated butanediol vapor in a hydrogen carrier over a supported copper catalyst at 230—250°C. The yield of butyrolactone after purification by distillation was about 90%. 

Furthermore, 60—100 L (14—24 gal) oil, having sulfur content below 0.4 wt %, could be recovered per metric ton coal from pyrolysis at 427—517°C. The recovered oil was suitable as low sulfur fuel. Figure 15 is a flow sheet of the Rocky Flats pilot plant. Coal is fed from hoppers to a dilute-phase, fluid-bed preheater and transported to a pyrolysis dmm, where it is contacted by hot ceramic balls. Pyrolysis dmm effluent is passed over a trommel screen that permits char product to fall through. Product char is thereafter cooled and sent to storage. The ceramic balls are recycled and pyrolysis vapors are condensed and fractionated. 

More advanced MHD power plants of the future are expected to use preheat temperatures of up to 2000 K. These temperatures, to be achieved by... 

Regenerative heat exchangers of both the fixed-bed and moving-bed types (67) have been considered for MHD use. The more recent efforts have focused on the fixed-bed type (68), which operates intermittently through recycling. A complete preheater subsystem for a plant requites several regenerators with switch-over valves to deflver a continuous supply of preheated air. The outlet temperature of the air then varies between a maximum and a minimum value during the preheat cycle. 

Economy of time and resources dictate using the smallest sized faciHty possible to assure that projected larger scale performance is within tolerable levels of risk and uncertainty. Minimum sizes of such laboratory and pilot units often are set by operabiHty factors not directly involving internal reactor features. These include feed and product transfer line diameters, inventory control in feed and product separation systems, and preheat and temperature maintenance requirements. Most of these extraneous factors favor large units. Large industrial plants can be operated with high service factors for years, whereas it is not unusual for pilot units to operate at sustained conditions for only days or even hours. 

The generated water vapor rises through a screen (demister) placed to remove entrained saline water droplets. Rising further, it then condenses on the condenser tube bank, and internal heat recovery is achieved by transferring its heat of condensation to the seawater feed that is thus being preheated. This internal heat recovery is another of the primary advantages of the MSF process. The energy performance of distillation plants is often evaluated by the performance ratio, PR, typically defined as... 

Raw material for dry process plants is ground in closed-circuit ball mills with air separators, which may be set for any desired fineness. Drying is usually carried out in separate units, but waste heat can be utilized directiy in the mill by coupling the raw mill to the kiln. Autogenous mills, which operate without grinding media are not widely used. For suspension preheater-type kilns, a roUer mill utilizes the exit gas from the preheater to dry the material in suspension in the mill. 

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