Photo: Martin Ljungqvist, the supervisor of the electrical department at Hydro Polymers, checking one of the flue gas fans in the boiler room which is controlled by Emotron variable speed drives.
Polyvinyl chloride or PVC it is the world’s second most widely used plastic with an annual production volume of more than 25 million tonnes. In the field of health care, it is the most popular material for gloves, tubing, blood bags and many other items. In the building industry PVC is used, for example, for pipes, cables, window frames and floor coverings.
Hydro Polymers’ PVC production plant is in Stenungsund, 50 km north of the Swedish city of Göteborg. This area has been the centre of Sweden’s petrochemical industry since the 1960s, largely because it is also the site of the country’s largest oil terminal and has excellent transport links. The PVC plant is the only one of its kind in Sweden and produces 210,000 tonnes of PVC every year. The company has 350 employees and an annual turnover of SEK 2 billion.
Multi-step chemical process
PVC production is a chemical process consisting of several stages.
The first step involves the electrolysis of salt water to produce, amongst other things, chlorine gas and sodium hydroxide. The sodium hydroxide is sold, primarily to the pulp and paper industry, while the chlorine gas is used in the next stage of the process where it reacts with ethylene and is transformed into vinyl chloride monomer (VCM). When the VCM molecules are linked together, a white power is produced.
Pumps, fans, blowers, mixers, mills and centrifuges, controlled by Emotron variable speed drives and softstarters, are used in the process. Emotron supplied the company with complete cabinets with the help of a local panel builder and everything is located in a central control room.
The plant’s boiler room, which produces steam used in the production process, contains two large boilers and three fans, a flue gas fan, a combustion air fan and a flue gas recirculation fan, all of which are controlled by Emotron variable speed drives. The vector brake function of the variable speed drives removes the need for brake choppers and braking resistors to stop the machinery quickly and safely.
Photo: Martin Ljungqvist is happy with the Emotron solution. ”Controlling the speed of the pumps instead of throttling the valves brings major benefits for us. It saves energy and reduces the wear on the machinery.”
Saving energy and reducing machinery wear
Emotron MSF softstarters are used to control the pumps, mills and fans in the PVC production process. Emotron variable speed drives control the speed of the mixers in the reactors where the particles form. The speed is crucial to ensure that the products have the right properties.
The next stage of the process involves drying the particles and, for some types of PVC, the particles are milled to create the end product which looks something like potato flour. After drying and milling the powder is transported to silos, where it is stored before being delivered to customers.
”In process stages where we have pumps running at slow speeds, the variable speed drives help to ensure that they operate as effectively and efficiently as possible,” says Martin Ljungqvist.
Emotron variable speed drives control the fans used to aerate and oxygenate the waste water in the company’s purification plant. PVC residues in the waste water are recovered and sold to customers, in particular in the floor covering industry. A new centrifuge has recently been installed in the purification plant to process the sludge. It is also controlled by an Emotron variable speed drive.
According to Martin Ljungqvist the variable speed drives offer major benefits.
”The biggest advantages of controlling the speed rather than throttling the valves are that we save energy and reduce wear on the motors. The number of variable speed drives will continue to grow as the plant’s machinery is gradually replaced.”