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Improvement of an Industrial Cement Precalciner Simulation Application of the Zone MethodHugh Jones Background and SignificanceMay (1999) developed a process simulation of an existing cement precalciner. The simulation calculated a number of operating parameters including the extent of calcination and gas and solids temperatures for a range of initial conditions. The model was developed for both plug flow and back mixed flow conditions. The radiative heat transfer was calculated using a correlation developed by Elsner et al.(1988). Aims and ObjectivesThe major aim of the project was to improve the accuracy and applicability of the simulation developed by May (1999). This was done by targeting the following two areas:
MethodMultiple CSTRs
The first improvement was to introduce the multiple CSTRs in series flow pattern to the simulation. The energy and conversion balances were solved assuming
Zone MethodThe Zone Method was then used to calculate the radiative heat transfer within
the calciner. This involved splitting the calciner into 10 surface and 10 gas
zones, which were assumed to be isothermal. First the direct interchange factors
and the total interchange areas were calculated between each zone. A temperature
profile along the calciner was assumed and then temperature dependent interchange
areas were calculated. Energy balances were written over each zone and solved
for gas temperature and calcination extent. If the calculated temperature profile
was sufficiently different from the assumed values, the last two steps were iterated
until the assumed and calculated profiles converged. Direct Interchange FactorsThe direct interchange factors, zizj, are the fraction of total energy leaving zone i being absorbed in zone j. They were calculated using the Monte Carlo method, where random numbers were used to determine emission angles and beam length. Depend on the gas absorption coefficient and the geometry of the precalciner. Total Interchange AreasThe total interchange areas were calculated from direct interchange areas as per Guruz and Bac (1981). They give the total energy interchange between each zone within the precalciner. They depend on the wall emissivities and the areas and volumes of each zone. Temperature Dependent Areas
Since the radiative interchange changes with temperature, the interchange areas must be adjusted for the temperature. These temperature dependent interchange areas were calculated using the total interchange areas and a temperature dependent weighting factor. The figure opposite shows an example of the surface to gas temperature dependent interchange areas Energy BalancesThe energy balances were written over each zone using the temperature dependent interchange factors to calculate the radiative heat transfer. The 20 simultaneous non linear equations were attempted to be solved using Newton's Method.Conclusions
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