Papers Delivered at International Conference on Cleaner Production
Beijing, China -- September 2001 -- Paper 24 of 30

Process control and Cleaner Production

By Zhou Zhongfan
Environmental expert for China-the Netherlands Cooperation Projects
(Chinese Research Academy of Environmental Sciences)

Abstract 

This lecture is focused on relationship of Process control with Cleaner Production. The level of process control in China’s industries is very low and effected the economic and environmental benefits of enterprises. Improved process control can be regarded as a part of the pollution prevention and resource & energy conservation. A case study of improving PC is also mentioned.

1. Background

China is a long history country and also is a most vigorous and dynamic developing country in world. Economic development in China is at full seed. Industry has been a major driving force to China’s economic growth over the last 20 years. But the problem of environmental pollution is along with it.

China’s top leaders are determined to reorient China’s development to strike a better balance between economic growth, social development and conservation of natural resources. China is an one of early made sustainable development agendas and have been promoted Cleaner Production (CP) in industries since 1993.

China’s production system is characterised by low technology production techniques, which are abundantly available all over the world. Simple and cheap production methods are easily copied and transferred from city to city and to countryside. Low efficiency of production and low quality products also effect enterprise’s competitive on the global markets.

Although mentioned as above points, high product quality with low product cost research & development based and high efficiency production with low energy consumption and emission are strongly interrelated. They need well-developed management systems, highly qualified, skilled and motivated staff, an efficient, well-developed technical equipment and process control system. To achieve this will be the major challenge and will require fundamental changes in China’s industrial production.

2. Cleaner Production is the only way to sustainable development for enterprises

Figure 1: Cleaner Production and Industrial sustainable development, Product modification, Input substitution, Technology change, Better process control, Equipment modification, Good housekeeping, Recovery/reuse on site, Employee training

Cleaner Production refers to the continuous application of an integrated preventive environmental strategy applied to processes, products, and services to increase eco-efficiency and reduce risks to humans and the environment. Cleaner Production aims at making more efficient use of natural resources (raw materials, energy and water) and reducing the generation of wastes and emissions at the source. This can be achieved in various ways as following:

The concept of Cleaner Production (CP), an international acceptable definition is as the following:

“Cleaner Production is the continuous application of an integrated preventative environmental strategy to processes, products, and services so as to increase efficiency and reduce the risks to humans and the environment"

CP options can reduce the material-, energy-, and/or water consumption per unit of product produced, and hence savings are made on the costs of these natural resources. Moreover amount and pollutant load of the various process waste streams (including solid waste, wastewater, air emissions) is reduced, and hence expenditures for treatment and disposal of waste and emissions are reduced. Most often the efficiency of production processes is increased as well, resulting in higher levels of production output, or improvement of the product quality.

3. Process control and with the prevention practices

Better process control is the important tool of achieved CP objectives. Improved process control can also be regarded as part of the prevention practice as “Process optimisation”. Improved process control is also related to other prevention practices, such as the improved production management, technology modification and equipment improvement or improving skills of the operators.

3.1 Process optimisation 

Process optimisation can be described as the modification of the production equipment and utilities in order to run the process under more optimal conditions, for example through the addition of measuring and control equipment. Optimisation of the installation by means of improved process control can lead to higher production figures and/or product quality and subsequently to higher revenues.

Optimisation using process control can result in the following:

• Maintaining process parameters to a preset value. When the actual value differs from the preset value, this is always observed by a control system and necessary actions can be taken automatically by the controller and the controlled element can be subsequently adjusted in a controlled way.

• Narrower ranges within a process parameter. The acceptable bandwidth in which the parameter can vary can be made narrower, because the control system can take quick and continuous actions.

• Faster response times to an off set situation. Off set situations, undesired situations in which the process is not running as it should, will be observed immediately (at least in case there are instruments to monitor the required parameters) so action can be taken immediately after such a situation occurs. Without a control systems the operator first has to observe the off set situation before action is taken.

• Reduction of the seriousness of the upset conditions during calamities. Because there can be a quick response to an off set situation and necessary actions can be controlled by the system as well, the possible damage or risks can be prevented or reduced.

So a process control systems can be more reliable, accurate, continuous, faster in response and without mistakes compared to a process controlled manually by operators.

These results from an improved process control again lead to an environmentally more safe and sound production:

• Lower raw material and energy consumption figures that reduce the operational costs;

• More constant and higher product quality;

• Higher process reliability;

• Reduction of potential hazardous circumstances and improved working circumstances.

A process can be optimised by introducing basic or advanced control strategies. Whether advanced control strategies are required depends on the complexity of the process. In practice a process is optimised according to the bottom-up principle, which means that first the process should be optimised by normal control and tuning. If this does not satisfy the required functionality one of the advanced control strategies might be considered. In case of large industrial enterprises having multiple installations, more optimisation in operation can be achieved in order to maximise the utilisation of an installation. Advanced scheduling and maintenance prediction techniques may be applied in process control systems and especially Manufacturing Execution Systems (MES) systems using specific scheduling and maintenance application tools.

3.2 Improving process control system can achieve the CP targets

Raw material and energy conservation

Process control systems provide the control functions to reduce energy and raw material consumption in installations. Some examples can illustrate this. For example process control related to furnaces, boilers and heat exchangers can improve the energy-efficient operation of these unit operations, i.e. less fuel is consumed and less steam is required.

Another way of energy-efficient operation is the use of frequency controllers on pumps or compressors. In this way, the equipment can run on the exact required quantities in stead of running at full speed and reducing the amount of transported medium by means of control valves. Furthermore process control systems can help to increase the yield of a product thus minimising the amount of raw materials and waste product per unit of (end) product.

Product quality increasing 

Product characteristics, such as constant product quality and high product quality become more and more important from an economic perspective. Process control systems provide the possibilities to run a process uninterrupted according to the same routines. Herewith it is easier to meet constant product specifications. Products with a higher and more constant product quality have a higher added value and generate more revenues, for example through a better customers relation, a lower amount of returned product or longer storage life.

Especially for the pharmaceutical and food industry, product tracking and tracing is more and more demanded to certificate product specifications. Process control systems provide the possibilities to product tracking and tracing. Increased product quality and reliability in relation with (consumer) quality standards can be useful assets for marketing of these products.

Process reliability improved 

Higher process reliability means less upset conditions and consequently fewer shutdowns. Especially under shutdown conditions product losses and hazardous situations occur. Thus by means of process control the process reliability can be improved leading to less waste material, less raw material and energy consumption, less off spec and re-work of product.

For example, production line start, stop and shutdown routines can be implemented in a process control system. These routines guarantee that the installation is started and stopped in the most optimum way, i.e. shortest time and least production losses. Furthermore these routines guaranty that starting and stopping procedures are always executed in a safe way.

Health and safety assured 

Monitoring and preventive actions on toxic emissions or flammable gasses can raise the health and safety situation for the employees of an enterprise and the community in the vicinity of the enterprise. It is often the case that the hazardous substances in a production process are also causing pollution when emitted or discharged.

3.3 Process control system is interconnected with other CP tools 

Improved Production Management 

As the implementation of process control system established, a tighter process control can contribute to an increased efficiency and optimised process, the skills of the management and the employees are improved. Also, not all CP options have to be related to more process control components, in terms of hardware and software. Also the use of an existing process control system, and the use of the data that become available can improve the efficiency of production and economic benefits in a company.

The management team should learn to use and incorporate the (improved) data acquisition in the management system of the company. Subsequently, the management of the company can be improved due to the improved information about the production process and the products. When the company becomes fully automated the work and the required skills of the operators will change and the operators and employees have to be trained for this.

Training, instruction and incentive schemes 

Once the process control of the company has been improved, the employees should be properly trained to use and maintain the equipment and the process and interpret the data that are generated by the process control system. When the process control of a company is improved, the company will probably not become fully automated immediately, but the approach will normally be bottom-up. This can mean that first instruments will be placed, and that the operators might have to register, interpret and act based on the information that the instruments supply. This does imply that new procedures have to be developed.

4. Case study

As the alkyl-benzene-sulphonic acid is a product produced here in a detergent workshop, and the sodium alkyl-benzene-sulphonic acid is the basic material to make the detergent powder, these two elements are vital to us and their quality will effect the benefits in this workshop. Besides, tail gas emission is large pollutants here, too. Therefore, the sulphonating facilities are selected to be the key instruments in CP audit activity.

This process flow chart for sulphonating instruments is as following:

SO3
Sulphonating
Alkyl Benzene
Tail Gas Treatment
Emissions neutralization
NaOH
Product forming
Air
Sulphur
Reactor

The process control improvement is as figure 2:

Operation control system

Figure 2

The benefits from improved process control system are:

• reduced SO2 emission from sulphur oxygen reaction and met the emission standards ;

• increased the efficiency of SO3 reaction, reducing the consumption of sulphur and increasing 20% of the product output;

• total cost of product is reduced more than 25%.