Coking model, coil

The coil coking model postulates a two-step mechanism ... 

The coil coking model has been tested in a number of plants. Table III presents these data. The agreement between the predicted and the actual run length is very good, which is a confirmation of the assumptions made in the development of the model. 

Likewise, Table XII reports a comparison of delayed coking models with industrial plant data for five different feedstocks covering a large range of different gasoil recycle ratios. The coil outlet temperature is in the range of 490°C and coil outlet pressure is about 5 ata. The results obtained confirm the reliability of this model. 

Coking in a commercial pyrolysis coil is a highly complex process which has not been modelled in precise mathematical terms.(H> 12,13) However, as is often the case, a simplified model utilizing certain empirically defined constants, has proven to adequately describe the coil coking phenomenon. 

The pyrolysis selectivity correlation and the coking models have been combined with momentum heat and mass transfer models to design pyrolysis coils. This application has led to the SRT III pyrolysis reactors which emphasizes low hydrocarbon partial pressure by employing a coil design with large diameter outlet tubes. 

Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

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