Part of the polyvinyl process
Responding to the need for sustainable production, Dr Detlev Keil, Project Manager, S-PVC licensing for Vinnolit in Ismaning, Germany, outlines methods employed in PVC manufacture.
Polyvinylchloride (PVC) is the third most produced polymer globally. It is used in Europe in up to 70% of durable construction applications. Pipes are made of PVC, as are window frames, films, sheets, cable sheathing, hoses/tubes, technical profiles, injection-moulded parts and blow-moulded components. Worldwide, an average consumption growth of four to five per cent is anticipated. The largest growth rates are expected in Asia.
The total capacity was 47Mt in 2010. While only about five per cent is for plastisol processing, the majority is processed as a thermoplastic with or without addition of plasticisers by extrusion, calendaring or injection moulding. This PVC is usually produced in a suspension process.
The PVC suspension process consists of three main steps – polymerisation, degassing and drying. A block diagram is shown below. The feed is liquefied vinyl chloride monomer (VCM), which is polymerised under pressure in an aqueous medium containing suspending agents and other optional auxiliary chemicals.
The solid polymer is formed by a radical chain mechanism. Initiators creating the starting radicals by decomposition due to heat influence start the reaction. The process is carried out discontinuously in stirred tank reactors under cooling at a temperature between 50-75°C. At a conversion of 85-90% the slurry is discharged from the reactor to a blow down vessel, serving as a buffer between the discontinuous polymerisation and the continuously operated downstream sections. Here, the majority of the unconverted VCM evaporates.
The residual VCM is removed by steam stripping in a stripping column and the degassed polymer is separated from the liquid phase by centrifugation and then dried. The evaporated unconverted VCM is pressurised and used again as feed, while the aqueous phase is waste water and sent to battery limit.
There are two aspects of the suspension process that require consideration – polymerisation heat removal from the reactor, and water consumption.
With the radical reaction mechanism, the polymer’s molecular weight, which determines the mechanical properties of the final products, is controlled by the polymerisation temperature. Polymerisation is carried out as an isothermal process, and heat of 1,550kJ/kg must be removed.
In fact, the reactor’s cooling capacity limits productivity. The chemical reaction’s rate can be adapted to the cooling capacity by the characteristics and quantity of initiator charged to the batch. This, however, is limited by the initiator price and PVC quality, such as thermal stability and colour of the PVC product.
A reflux condenser can increase a conventional reactor’s cooling capacity with a cooling jacket outside, such as a semi-circular cooling helix on the inner shell. In the graph below, the wall construction for the conventional design is shown on the left side, and the high performance (HP) reactor on the right side. The considerably smaller wall thickness with the inner cooler design enhances heat conductivity.
In the HP reactor, the cooling water coils are inside the thick load-bearing wall. Instead of the typical 27mm wall thickness, the resistance to heat conduction through the wall is only 3.6mm. Due to improved conduction, the overall heat transfer coefficient is significantly increased alongside the cooling surface, due to the semi-circular shaped cooling coils. Both effects contribute to the increase in performance compared with a conventional reactor (see table, below). During 10 years of industrial experience it has been proven that the wavy surface is suitable for closed and clean reactor operation, as well as a conventional flat surface.
With the HP reactor, about three cubic metres of fresh water are required to produce one tonne of PVC.
After VCM-stripping, the solid PVC is separated from the aqueous reaction medium by centrifugation, but a small fraction of the fine PVC particles remains in the water phase. Such water, if again used in polymerisation, degrades PVC quality, and can only be used for flushing purposes. But the demand for flush water in the downstream units is small compared with the water demand for polymerisation. For a considerable reduction in water consumption, an appropriate purification technology must be employed.
An approach to overcome this problem is offered by modern membrane technology. In a screening process, membranes made from different materials and designs of apparatuses have been tested. A certain type of membrane for ultrafiltration has been found to provide the required degree of solids removal. The outflow from the centrifuge is fed through the membrane elements, whose pores are large enough to allow water transition, but small enough to retain the PVC particles. Molecules or ions present in the centrifuge waste water do not cause a problem. In the block diagram, above the circulation of water in a PVC suspension, employing water recycling, has already been incorporated.
The development of this process was supported by funds from the EU in project LIFE06 ENV/D/000470. It was conducted at Vinnolit’s S-PVC plant at the Knapsack site in the Cologne area. The main image (top) shows a photo of the production-scale ultrafiltration unit installed at this plant.
Fifty per cent of the water for the production of pipe-grade PVC is replaced by recycled water. So the specific water consumption has been reduced to 1.4m3/t. Since 2007, more than 250,000m3 of water is saved every year at the Knapsack site. Efforts are being made to increase the amount of recyclate up to 75%, by expanding the use of ultrafiltration to other PVC grades.
Considering the cost for fresh demineralised water and waste water treatment at the Knapsack site, the project is profitable if the life time of the membranes exceeds one year. Flux through the membranes can be preserved on an acceptable level by regular cleaning operations for approximately two years (see below).
Dr Oliver Mieden, Environmental Affairs & Corporate Communications Manager. Vinnolit GmbH & Co. KG, Carl-Zeiss-Ring 25, 85737 Ismaning, Germany. Tel: +49 (0)89 96103-282. Email: firstname.lastname@example.org Website: www.vinnolit.com Thanks to co-author Jörg Hessberg, from Vinnolit.