Cementing success

In contrast to the commonplace nature of its end product, the cement industry has long been in the forefront of adopting the latest process control methodologies to improve the performance of its highly energy-intensive plants.

One of the earliest industrial applications of a fuzzy-logic approach to control, for example, was taken up by the industry in the early 1980s. This was in the form of the LINKman expert control system which, in its latest manifestation, is still a popular choice among cement producers, as the sidebar describes.

Recently, however, another approach to advanced process control (APC) - one more familiar to other major process industries such as petrochemicals and oil refining - has begun making inroads into the control rooms of cement factories around the world. This is the model-based predictive control system of APC specialist Pavilion Technologies.

Typical of the operations to have benefited from Pavilion's approach is that of Golden Bay Cement, New Zealand's largest cement manufacturer and supplier. With an annual production of 600 000tonnes, the company's Whangarei plant on the North Island is the country's only dry process cement plant.

Like many manufacturers, Golden Bay was faced with growing demand for its cement products and needed to increase the plant's overall production efficiency to boost output. In setting the goal of increasing product throughput while reducing residue variation, the Portland plant came to the conclusion that the expert control system it had been using could not deliver the results it needed on the cement mills. The site therefore chose to upgrade its control systems to help produce more cement, improve quality and lower operating costs.

After reviewing available APC systems and service offerings on the market, Golden Bay opted for Process Perfecter, Pavilion Technologies' multivariable predictive control and optimisation software. The Perfecter package combines Pavilion's patented steady-state optimisation and model predictive control with a variable dynamics capability to deliver a solution capable of controlling quality targets, managing multiple production goals, and rejecting disturbances.

According to Pavilion, because it is based on a dynamic model of complex processes - such as those to be found in a cement mill - Process Perfecter allows for more robust real-time control and optimisation than the traditional static, expert system-based solutions. By modelling the non-linear behaviour of a process as well as the interdependencies of key process parameters, it helps its customers 'to realise significant operational efficiencies and related cost savings'.

This was certainly the case at Whangarei, where Golden Bay and Pavilion engineers worked in partnership to install Process Perfecter in the plant's cement mill six, an open circuit mill where the final process stage takes place - grinding the clinker from the cement kilns to produce the finished product.

In open circuit mills, coarse material passes through the system once, often producing various particle size distributions and thus residue variation. Process Perfecter, with its predictive capabilities, anticipates cement residue variations caused by measurable process disturbances.

It continuously updates its predictions based on the internal models of future behaviour, process interdependencies and actual performance. In this way, says Pavilion, it can integrate the diverse and conflicting aspects of the grinding process and perform actions to meet Golden Bay's quality and efficiency objectives.

By combining recent historical process conditions with expected future performance in this way, Process Perfecter makes decisions like an experienced operator. But with Perfecter, the combinations and levels of actions under Golden Bay's control are automatically applied to the mill process, maximising the plant's operational efficiency.

This, in Pavilion's view, provides a significant advantage over less sophisticated technologies, which rely purely on past and current conditions without forecasting future performance or evaluating the inter-relationships between process parameters.

Process Perfecter was implemented in just four weeks on mill six and immediately began delivering a significant return on investment. The mill showed a 3% increase in throughput and a 50% reduction in residue variation. More significantly, perhaps, the software also enabled Golden Bay to realise another key goal - fully automatic control of its mill.

Confident of Perfecter's performance, the company shifted its control paradigm from 'open loop' advisory control on just residue, to full closed-loop control. This means the solution is continuously improving and driving the process to achieve its goals each day of production. Golden Bay's operations manager, David Walker, says: 'We are delighted that Pavilion's APC solution is delivering to our expectations with a return on investment of less than year.'

SIDEBAR: Experts in cement 'auto-piloting'

Such is the claim made by ABB for the latest version (Version 4) of its OptimizeIT Expert Optimizer advanced control system. The system is in effect the next generation of the established expert-system based LINKman solution and builds on years of experience in providing advanced control for the cement industry - experience that embraces worldwide installations on more than 170 kilns and 70 mills.

Introduced last year, Version 4 incorporates a new set of process control modules alongside enhanced versions of the existing optimising tools and a 'Neuro-Fuzzy' module. A common user interface now offers seamless interoperability with other ABB IndustrialIT products and systems. Increased profit for the manufacturer, says ABB, stems from greater productivity, optimised control of waste fuels, longer refractory life, and lower grinding costs as a consequence of reduced over-burning of clinker.

The system was developed in close cooperation with several ABB customers to ensure it would meet the real, day-to-day needs of the industry. 'Our impressions are very good,' says Krit Bunnag of Thailand's Siam Cement Industry.

'Expert Optimizer is very user-friendly, with a clear flow of data that makes it easy to understand.' Siam Cement has installed the system on lines 4 and 5 of its Kaeng Khoi plant with the expectation of increasing average output from existing kilns by between 100 and 150tonnes per line per day.

Holcim was equally impressed. Dr Moser, plant manager of Holcim Altkirch in France, which had used LINKman Version 3 for five years, ran his plant on 'auto-pilot' 97% of the time during tests on Expert Optimizer.