Cracking heat transfer rates

In catalytic cracking, a large amount of heat needs to be supplied at the reactor inlet to vapourize the feed and provide the heat of reaction. In commercial units, this heat is provided by the hot catalyst recirculated from the regenerator. High heat transfer rates are achieved when the fluidized catalyst is mixed with the feed. In some experimental units, feed and catalyst are injected at reactor temperature. The heat of reaction must then be supplied by an external heating element, at much slower rates of heat transfer. The product selectivity from such laboratory units cannot be expected to simulate that of commercial units... 

Great attention should be paid to the convection section. For each feed stock pair, different steam dilution, different initial cracking point, different physical properties, and different hp-steam production all result in different quantities of heat which have to be transferred in the convection section bundles. Table II shows typical heat transfer rates for a... 

Table II. Relative Heat Transfer Rates in a Naptha/AGO Flexible Cracking Furnace0...

The weight of catalyst in a vessel is determined by measuring the pressure differential between taps installed at the top and bottom. Density of the fluidized catalyst is determined in a similar manner from the differential pressure between taps located a measured distance apart in the dense phase. Location of the catalyst level can be determined from the combination of the density and the total weight of catalyst, or by the use of a series of pressure taps placed at intervals along the height of the vessel. A hot-wire probe has been used to locate the level in laboratory fluidized beds (250), but this technique has not been adopted for fluid cracking units. The method depends upon the fact that heat-transfer rate from the heated wire is much higher when immersed in the dense phase of fluidized solids than when in the dilute phase. 

Initial investment for horizontal tube evaporators is low, but heat transfer rates may also be relatively low. They are well suited for non-scaling, low viscosity liquids. For several scaling liquids, scale can sometimes be removed from bent-tube designs by cracking it off periodically by shockcooling with cold water or, removable bundles can be used to confine the scale to that part of the heat transfer surface which is readily accessible. 

The corrosion rate of nickel in sodium hydroxide is adversely affected by heat transfer by small amounts of oxidisable alkaline sulphur-containing salts, e.g. Na2SOj, NajS Oj, Na S and, at high temperatures, by alkaline oxidising agents, viz. NaClOj and NajOj. In the former circumstance Alloy 600 is more resistant than nickel, but not in the latter. When Alloy 600 is used for service in caustic alkalis, it should be stress relieved after fabrication to minimise the possibility of stress-corrosion cracking. 

All physical changes of surface or subsurface that effect local rate of heat transfer Slump, liner separation, sub-surface voids, surface cracks Requires internal traveling mechanism... 

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