Coking furnace radiant tubes

Besides the high temperature corrosion processes (such as oxidation, sulfidation and ash deposition) that occur on the outer wall of furnace radiant tubes, high temperature sulfidation, naphthenic acid corrosion and coking on... 

High temperature oxidation of coking furnaces (furnace radiant tubes work at about 500X)... 

Run Length, Coke is produced as a side product that deposits on the radiant tube walls. This limits the heat transfer to the tubes, and increases the pressure drop across the coil. The coke deposition not only limits the heat transfer, but also reduces the olefin selectivity. Periodically, the heater has to be shut down and cleaned. Typical mn lengdis are 40 to 100 days between decokings. Prediction of mn length of a commercial furnace is stiU an art, and various mechanisms are postulated in the literature. Coke also deposits in transfedine exchangers. Mechanisms for coking in radiant coils and transfedine exchangers appears to be different for different feeds. 

Losing flow for a few seconds won t significantly upset downstream equipment, provided one has taken the precaution to bypass any low flow trips. The exception to this is the radiant tubes in a furnace, which might coke up or overheat quickly when flow is lost. 

The primary reformer is a process furnace in which fuel is burned with air to provide the heat of reaction to the catalyst contained within tubes. This area of the furnace is usually referred to as the radiant section, so named because radiation is the primary mechanism for heat transfer at the high (600-700°C) temperatures required by the process. Reforming pressures in the range 3-4 MPa in the reactor provide a reasonable compromise between costs and downstream recompression requirements. Carbon formation (coking) in the primary reformer must be prevented (as is discussed further in paragraph 4). 

A cross section of a furnace tube for one small segment of the numerical integration is shown in Figure 3. An assumption is made for this model that the radiant temperature. Trad has a constant value for each cracking zone. The coke thickness as well as the metal thickness is specified. This model, as presented, does not include any techniques for forecasting coke buildup on the inner tube wall. The fluid film thickness is calculated from an ideal film model where... 

As coke is deposited in the oil film, the temperature in the tube rises, and it continues to rise as coke is deposited until finally the tube attains the temperature of the furnace or it bursts. For normal thicknesses of coke, the amount of heat absorption is not decreased greatly. Partridge and White find that a normal thickness of boiler scale does not decrease the rate of radiant-heat absorption by more than about 3 per cent. In general, coke deposits act in the same manner. 

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