Pebble heater

Convective Heat Transfer Eqnipment nsiug the trne convective mechanism when the heated particles are mixed with (and remain with) the cold particles is nsed so iufreqneutly that performance and sizing eqnatious are not available. Snch a device is the pebble heater as described by Norton (Chem. Metall. E/ig., Jiily 1946). For operation data, see Sec. 9. 

Fields of Application One of the major advantages of the gravity-bed technique is that it lends itself well to true intimate counter-current contacting of solids and gases. This provides for efficient heat transfer and mass transfer. Gravity-bed contacting also permits the use of the sohd as a heat-transfer medium, as in pebble heaters. 

Fixed beds are the main interest of this Section. Usually it is adequate to assume that the fluid and solid are at the same temperature at a point. There are cyclic processes, however, where the solid is first heated with flue gases or by burning off carbon before contacting the reacting fluid for a time. A moving bed of heated pebbles (Phillips pebble heater) has been used for the production of olefins from butane and for the fixation of atmospheric nitrogen. A fluidized sand cracker for the production of olefins functions similaiiy, with burning in a separate zone. 

The fixed beds of concern here are made up of catalyst particles in the range of 2-5 mm dia. Vessels that contain inert solids with the sole purpose of improving mass transfer between phases and developing plug flow behavior are not in this category. Other uses of inert packings are for purposes of heat transfer, as in pebble heaters and induction heated granular beds—these also are covered elsewhere. 

The pebble heater, Figure 17.28(b), is used in the same manner its application to the pyrolysis of oils to make ethylene also did not prove competitive and has been abandoned. 

Fig. 11. Pebble heater for heating steam to temperatures impractical in metallic units. Also used for heating air. hydrogen, methane, and other gases for processing purposes. In reverse, a pebble healer may be used to recover heat from hot gases. The pebbles are heated in top chamber. 4 by direct contact with combustion gases and passed through a throat to lower chamber B. where heat is transferred to cool gases. The two chambers are maintained at the same temperature so that there will be no gas flow between them. An average cycle on the pebbles is 30-50 minutes...

FIG. 19-13 Noncatalytic gas-phase reactions, (a) Steam cracking of light hydrocarbons in a tubular fired heater, (b) Pebble heater for the fixation of nitrogen from air. (c) Flame reactor for the production of acetylene from hydrocarbon gases or naphthas. [Patton, Grubb, and Stephenson, Pet. Ref. 37(11) 180 (1958).] d Flame reactor for acetylene from light hydrocarbons (BASF), (e) Temperature profiles in a flame reactor for acetylene (Ullmann Encyclopadie der Technischen Chemie, vol. 3, Verlag Chemie, 1973, p. 335). 

Figure 17.28. Reactors with moving beds of catalyst or solids for heat supply, (a) Pebble reactor for direct oxidation of atmospheric nitrogen two units in parallel, one being heated with combustion gases and the other used as the reactor (Ermenc, (1956). (b) Pebble heater which has been used for making ethylene from heavier hydrocarbons (Batchelder and Ingols, 1951). (c) Moving bed catalytic cracker and regenerator for 20,000 bpsd the reactor is 16 ft dia, catalyst circulation rate 2-7 Ibs/lb oil, attrition rate of catalyst 0.1-0.5 Ib/ton circulated, pressure drop across air lift line is about 2psi (L. Berg, in Othmer, 1956).

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