Improvement of an Industrial Cement Precalciner Simulation Application of the Zone Method

Hugh Jones
Supervisors: Prof K.D. King, Dr D. Desai

Background and Significance

May (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 Objectives

The 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:

• Extend the flow pattern to include multiple CSTRs in series
• Introduce the Zone Method to accurately calculate the radiative heat transfer.

Method

Multiple 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

• Each CSTR section was fully mixed
• Inlet conditions for each section were set by the outlet conditions from the previous section
• The effect of the number of CSTRS on calcination was investigated for a number of initial conditions. The results were compared to those calculated using the back mixed and plug flow models. The Figure opposite shows the extent of calcination with respect to the number of CSTRs in series for the default conditions of the simulation.

Zone Method

The 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.
The gas properties were approximated by a three grey gas model using the method described by Jenkins and Moles (1981). The overview of the calculation method used is shown.

Direct Interchange Factors

The 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 Areas

The 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 Balances

Conclusions

• The extent of calcination increased with the number of CSTRs modelled. The calcination approached the value calculated using the PFR model but did not reach it.
• Radiative heat transfer from each gas zone was primarily intrazone, with some transfer to gas and surface zones up to 2 away.
• Radiative heat transfer from surface zones was primarily to adjacent gas zones with some transfer to gas zones up to 3 away.
• The energy balances could not be solved using the code written. This may be due to the difficulty of solving for both calcination and temperature, being three orders of magnitude different, or due to errors in the code.
• The primary aim of further work on this area would be to achieve convergence of the energy balances. Increasing the number of zones and the complexity of the fluid flow model within the zones would also add to the accuracy of the simulation.