(191f) S.R.C.- Super Radiant Coil: the Step Ahead for Improved Steam Cracking Technology

The trend in ethylene production is to improve the olefins selectivity of cracking coils to compensate for the increasing cost of feedstock. This requires small bore tubes, resulting in a short cycle time between decokings. The Super Radiant Coil (SRC), briefly described in this paper, combines the advantage of short residence time with the advantage of large bore tubes, reducing the tube wall temperature and the coking rate. The production of ethylene by steam cracking of hydrocarbons is a very mature technology. During the past five decades Engineering efforts focused on the pyrolysis olefins selectivity by reducing both the gas residence time and the hydrocarbon partial pressure and by increasing the fluid temperature to achieve higher yields. The resulting high heat flux and tube metal temperature force the radiant coil to operate close to its metallurgical limit. The consequences are high a coking rate, creep and carburization. Furnace designers are continuously developing the geometry of the radiant coil in order to improve the convection heat transfer coefficient and obtain a higher surface to volume ratio. In the existing ethylene furnaces the heat is transferred from the fire-box to the process fluid, through the tube wall, by three transport mechanisms: radiation, conduction and convection. The heat transfer in conventional cylindrical tubes mainly combines three steps: a) radiation from the firebox to the outside tube surface b) conduction through the tube metal wall c) convection from the inner tube wall to the process gas

Many solutions have been proposed to improve the heat transfer rate using inside finned tubes to increase the heat transfer surface or the use of tubes with spiral mixing elements. The above techniques focus exclusively on improving the tube side heat transfer by convection. The Super Radiant Coil (SRC) The Super Radiant Coil concept is based on a creative use of several fundamentals of heat, momentum and mass transfer, combined in order to enhance the heat transfer to the gas and control the residence time in the cracking tube. Considering the relatively high tube metal temperature (up to 1125 Deg. C) and the gas temperature in the range from 600 to 900 Deg. C , radiative heat transfer to the gas would be high since the heat flux is proportional to the difference of fourth powers of absolute temperatures. Actually, only a few percent of the heat is transferred to the process gas directly by radiation because the gas is almost transparent and the beam length is too short. The scope of the new technology is: a) maximise the use of radiative heat transfer inside the tube. b) optimise the tube geometry to control the residence time at a predefined (low) value without the constraint of small bore tubes. c) obtain a lower tube metal temperature compared to the cylindrical geometry, in order to reduce coking rate and carburization. d) control the gas pressure drop in an acceptable range. All these features lead to optimum furnace run length and tube life with high olefin yields. The new cracking tube technology consists of a conventional cracking tube equipped with an internal special device. This arrangement shows the following advantages: a) Optimise the residence time at a given cracking pressure and temperature. Short residence time can be obtained even by using large bore tubes taking advantage of a proper selection of the internal special device. b) Enhance the heat transfer. Radiant heat is transferred from the inner surface of the tube to the internal special device. This is then integrally transferred to the gas by convection. c) The reduced tube section, generated by the internal device, enhances the convection heat transfer of the inner surface of the tube. The main result is a lower metal temperature being a portion of heat delivered to the gas by means of an added radiative transfer area. The improvements of the SRC technology can be summarised as follows : a) The ratio of effective heat transfer surfaces to reactor volume is increased. b) Radiative heat is transferred to the internal device and then by convection to the gas, i.e. both inner and outer surfaces are active and effective. c) The convection heat transfer is improved by a better gas velocity profile. The performance of the Super Radiant Coil applications and the results are outlined on the following pages. This technology is being tested at the Ethylene Plant owned by Polimeri Europa and located in Gela, Italy and the initial results, after four months of testing, are extremely encouraging.