The Benefits of Automated Process Control in Cement Production

US-based company Vulcan Materials is a major producer of aggregates-based construction materials, including asphalt and ready-mixed concrete. A couple of years ago, the company installed an Insitec Fineness Analyzer from Malvern Panalytical and an Advanced Process Control (APC) solution from Pavilion Technologies, a Rockwell Automation company, to upgrade its cement finishing circuit.

These solutions helped the company to transform its entire operation and provided significant economic gains, especially in terms of improved product quality and energy consumption. This article describes the project, assessing the major benefits delivered by these advanced systems.

Manual Approach of the Finishing Circuit

Before the installation of automated control, the performance of the finishing circuit was maintained by manually exploiting the feed rate, separator draft, and separator speed as these parameters have a tremendous impact on process behaviour. Regular off-line Blaine analysis and routinely-measured plant variables provided information on which to base remedial action.

In the cement industry, Blaine measurements are extensively utilized to measure particle fineness. The analysis, which is based on ascertaining the pressure drop across a packed bed, can only be carried out off-line. Figure 1 shows a diagram of the Vulcan materials cement finishing circuit.

Schematic of the Vulcan Materials cement finishing circuit.

Figure 1. Schematic of the Vulcan Materials cement finishing circuit.

At Vulcan Materials, sampling and related measurement is performed every two hours to check the quality of products. Although the data can be utilized for process control, it does not provide real-time insight into the process. This manual approach presented a lot of limitations and consequently an automated control solution was installed to improve operation. This provided a means to enhance product quality and simultaneously reduced energy consumption. Vulcan Materials selected the Pavilion control package, which is specifically designed for optimising cement finishing circuits.

Model Predictive Control Technology

A multivariate process model, the core of the Pavilion Cement Grinding Application solution predicts performance from a variety of inputs. Automatic exploitation of controlled variables on the basis of these predictions keeps the plant within a defined operating window to optimise goals. The aim was to maintain product quality within specifications, improve operational stability, reduce variability, and improve fresh clinker feed rate subject to equipment constraints.

A control scheme uniquely customized to the plant was developed to capture the specific details of the Vulcan Materials circuit. The configuration process defines the mathematical link between different parameters for the individual unit, creating a precise model for continuous and dynamic optimisation. When the plant is operational, the model works in real time and drives the system towards an optimal operating point within the defined limits. Then, a feedback loop determines process response and compares actual response to preferred values.

A steady state optimiser measures the exact changes to manipulated variables so as to drive controlled variables to target. Following this, a built-in controller projects these into the future, forming a range of control actions for individual manipulated variables and predicting the effect on the controlled variables to satisfy multiple targets concurrently. The aim is to reduce a specific objective function so that the predicted process outputs are close to the preferred reference trajectories.

This optimization process is performed again as process values are re-read and the impact of the initial control action has been noted, so that the future prediction horizon period is sustained. This is called as real-time receding horizon control.

To sum up, the system is an integrated controller and optimizer designed to calculate and drive the process to the optimal position, as it alters, in this case, every 30 seconds. The Pavilion controller exploits both separator speed and clinker feed rate to control product Blaine. Figure 2 shows the screenshot of the Pavilion mill solution.

Screenshot of the Pavilion8 mill solution

Figure 2. Screenshot of the Pavilion8 mill solution

The table below illustrates the controlled variables for the circuit as they were first set up.

Table 1. Finish mill controlled variables.

Description Control type
Blaine Desired target
Elevator kW Upper constraint
Mill outlet material temperature Upper / Lower constraint
Mill condition Desired target
Mill kW  
Return flow  

Correlations between Cement Performance and Particle Size

One of the disadvantages of Blaine is that it offers a single number representation of a sample instead of the distribution provided by particle size measurement. Figure 3 demonstrates a link between 1-day strength and the quantity of material between 3 and 30µm. Particle size data was produced using Malvern Panalytical’s off-line laser diffraction analyzer, Mastersizer 3000.

Correlation between 1-day strength and the fraction of material between 3 and 30µm.

Figure 3. Correlation between 1-day strength and the fraction of material between 3 and 30µm.

Although two cement samples with different particle size distribution offer the same Blaine, they perform differently in the field owing to the relative proportions of over-sized material. Assessing the link between cement performance and particle size gave further insight for enhancing product quality.

Real-time Particle Size Measurement

On-line installation of an Insitec Fineness Analyzer.

Figure 4. On-line installation of an Insitec Fineness Analyzer.

An online Insitec Fineness Analyzer was installed by Vulcan Materials that produces real-time particle size data. The analyzer was easy to install and runs reliably round the clock and it has now become an integral part of the plant. A representative part of the process stream flows endlessly via the sample loop returning to the line after analysis (Figure 4). Measurement was also fast and non-destructive and the system produces up to four complete particle size distributions every second. In addition, the analyzer software enables integration with the plant control platform, completely automating all aspects of measurement.

Conclusion

The Insitec Fineness Analyzer and the Cement Grinding Application solution allowed Vulcan Materials to considerably reduce the cost of cement production and reap the benefits of automated control and real-time monitoring.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.

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