Fluidised bed process

Fluidised-bed techniques, pioneered with low-density polyethylene, have been applied to PVC powders. These powders can be produced by grinding of conventional granules, either at ambient or sub-zero temperatures or by the use of dry blends (plasticised powders). The fluidised bed process is somewhat competitive with some well-established paste techniques, and has the advantage of a considerable flexibility in compound design. 

Polyolefin copolymers Although there is a wide variety of these available, the only one currently commercially available as a compounded powder is saponified EVA. This is reported to have good weatherability and will not suffer from ESC. One major advantage this coating has is that it can be applied by the fluidised bed process at low temperatures and this offers the possibility of coating temperature-sensitive metals such as galvanised steel. 

Nylon 11 is applied using a fluidised bed process to a wide variety of substrates including metal chair frames, door furniture and wire dishwasher baskets. It can also be applied by electrostatic spraying, but generally only where the application is decorative and where the metal work is thin, i.e. less than 0.2 mm. 

The operational window of a fluidised bed process is defined by the minimum fluidisation velocity, Uml, at which a settled bed of adsorbent beads starts to fluidise and the terminal velocity ( /t) at which the bed stabilises and adsorbent beads are entrained from the bed. 

What development and progress has occurred during the last ten years or so Experience has accumulated and a number of fluidised bed processes are operated successfully. Industrial companies are reluctant to disclose too many details of a successful and productive reactor but I have seen beds as large as 17 m in diameter very uniformly and stably fluidised which could be shut down and re-started without trouble. 

Table 8 Operating conditions employed in fluidised bed processes...

A fine spherical silica-based catalyst with the physical characteristics reported below is being investigated for the development of a new fluidised bed process. Fundamental parameters such as the bed voidage and minimum fluidisation velocity, umf, have to be determined. 

The role of gold has now emerged as having greater significance than was realised at the outset of these operations. Most of the commercial processes are fixed-bed, but at the end of 2001, BP commissioned the brand new plant in Hull, UK. This is the world s first fluidised-bed process for VAM, while 80% of today s VAM plants worldwide are more than 20 years old and use a fixed-bed process.44... 

Moving from a fixed to a fluidised-bed operation also required a new catalyst, and the one selected was a supported gold palladium system in the form of very fine spheres, prepared in collaboration with Johnson Matthey. Hence, gold-based catalysts are being used for this new fluidised-bed process, and are well established in fixed-bed processes for the large-scale manufacture of VAM. 

W. Kaminsky and H. Sinn, Pyrolysis of plastic wastes and scrap tyres using a fluidised bed process.In J. L. Jones and S. B. Radding(eds) Thermal Conversion of Solid Wastes and Biomass.ACS Syposium Series 130, American Chemical Society, Washington,D.C (1980). 

Fast pyrolysis occurs in time of few seconds or less. Therefore, not only chemical reaction kinetics but also heat and mass transfer processes, as well as phase transition phenomena, play important roles. The critical issue is to bring the reacting biomass particle to the optimum process temperature and minimise its exposure to the intermediate (lower) temperatures that favour formation of charcoal. This objective can be achieved by using small particles, thus reducing the time necessary for heat up. This option is used in fluidised bed processes that are described later. Another possibility is to transfer heat very fast only to the particle surface that contacts the heat source. Because of the low thermal conductivity the deeper parts of the particles will be maintained at temperatures lower than necessary for char production. The products that form on the surface are immediately removed exposing that way consecutive biomass layers to the contact with the heat source. This second method is applied in ablative processes that are described later. 

Thermal conversion of biomass was perfomied in a Pilot Demonstiation Unit (PDU) scale fluidised bed reactor with a throughput of 5 kg biomass/h and in a laboratory fluidised bed reactor 1 g/batch. Therefore the Hamburg Fluidised Bed Process" ( Hamburger Wirbelschichtverfahren ) had to be modified and empirically optimised for biomass feedstock. 

It is applied using a fluidised bed process, generally in two coats. A precompounded blend of p.v.d.f., corrosion-inhibitive pigments and adhesive components is applied first, followed by a top coat of pure p.v.d.f. The primer coat protects the metal and the top coat protects the primer coat from attack by the chemicals. 

Pickering SJ, Yip H, Kennerley JR, Kelly R, Rudd CD, The recycling of carbon fibre composites using a fluidised bed process, FRC2000 Conference, Newcastle, Sep 13-14, 2000. 

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