Dynamic modelling of catalytic SO2 converter in a sulfuric acid plant of an industrial smelter
Date
2018-04-16
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Abstract
In industrial nickel and copper production, sulfur dioxide (SO2) is generated from the
combustion of sulfide ores. With increasingly tightened regulations on SO2 emissions, a sulfuric
acid plant has become a crucial part of industrial smelters. It converts environmentally harmful
SO2, which is generated in smelter furnaces, roasters, and Cu-reactors, into commercially
beneficial sulfuric acid. This method is recognized as one of the most effective ways to ensure
that smelters are able to satisfy the SO2 emission regulations.
A sulfuric acid plant is primarily comprised of a central catalytic SO2 converter, SO3 (sulfur
trioxide) absorption towers and a series of interconnected heat exchangers. The catalytic SO2
converter is the key component and the focus of this research. Both steady-state and dynamic
models of the converter are developed in this thesis.
A steady-state model of the converter is established in accordance with steady-state
mass and energy balances. The developed model provides an explicit relation between SO2
conversion ratio and gas temperature, which is denoted as the heat-up path of the converter.
By combining the heat-up path with the equilibrium curve of the SO2 oxidation reaction, an
equilibrium state for every converter stage can be obtained. Using the developed steady-state
model, simulations are performed to investigate the effect of inlet SO2 molar fraction and gas
temperature on the equilibrium conversion ratio.
In an industrial SO2 converter, the SO2 concentration and conversion ratio out of each
bed are important variables but are not measured in real time. To monitor these unmeasured
variables in industrial operations, a soft sensor is proposed by combining the derived steadystate
model with dynamic data analysis. The obtained soft sensor provides a real-time
estimation of outlet SO2 concentration and the conversion ratio from measured temperatures.
For synchronization between the inlet SO2 concentration and outlet temperature, a first-order
exponential data filter is applied to the feed SO2 data. With the filtered signal being used,
the proposed soft sensors give a satisfactory estimation of both outlet SO2 concentration and
conversion ratio in the converter stages.
Dynamic modelling is carried out using two different model forms: ordinary differential
equation (ODE) and partial differential equation (PDE) models. The ODE model is obtained by
applying dynamic mass and energy conservation to the SO2 converter. The resulting model can
be used in industrial applications and describes the converter performance even if information
of reaction kinetics is not available. A good fit with collected industrial data verifies the validity
of the developed ODE model. The effect of process input variables is studied using simulations
with the ODE model.
Dynamic modelling is performed by implementing mass and energy balances on both fluid
and solid-phase gas flows. The proposed two-phase dynamic model, which takes the PDE
form, is able to generate detailed profiles of the SO2 converter within time and space. With the
estimated parameters, this two-phase dynamic model generates a good fit between the simulated
and measured outlet temperatures. Based on the PDE model, simulations are run to investigate
the detailed mechanistic performance of the converter. The detailed PDE model provides useful
explanation of, and prediction for the converter behaviour.
Description
Keywords
dynamic modelling, catalytic SO2 converter, sulfuric acid plant, industrial smelter