Cracking fuel contaminants from

In the 1970 s, heavy fuel came mainly from atmospheric distillation residue. Nowadays a very large proportion of this product is vacuum distilled and the distillate obtained is fed to conversion units such as catalytic cracking, visbreaking and cokers. These produce lighter products —gas and gasoline— but also very heavy components, that are viscous and have high contaminant levels, that are subsequently incorporated in the fuels. 

The Cj plus bottoms from the demethanizer then go to the deethanizer. A propylene-propane bottoms product containing 90-92% propylene is obtained which may either be sold, used directly as propylene- 90, or further purified. The ethylene-ethane overhead from the deethanizer is separated in the splitter tower yielding a 99.8% overhead ethylene product at -25°F. The ethane bottoms at -l-18°F may either be sent to fuel gas or used as feed to an ethane cracking furnace. Overall ethylene recovery in these facilities is about 98%. The product is of very high purity with less than 50 parts per million of non-hydrocarbon contaminants and a methane plus ethane level below 250 ppm. 

As well as direct incineration of plastics waste, imder clean and efficient conditions, there has been some research into conversion into fuel oil, by chemical methods. The Veba Combi-Cracking (VCC) process produces synthetic crude oil under liquid phase hydration. In trials, 100 tonnes of mixed and contaminated plastics waste from normal domestic sources was hydrated to high quality oil, similar to that used as a source for diesel fuel. With metal-free granular material, costs were estimated at about DM 500/tonne. The process has also been used with polyurethane waste, producing oil that can be mixed with new oil (but costing over twice). 

Use of a steam generator to separate the primary loop from the secondary loop largely confines the radioactive materials to a single building during normal power operation and eliminates the extensive turbine maintenance problems that would result from radio-actively contaminated steam. Radioactivity sources are the activation products from the small amount of corrosion that is present in the primary loop over the 12-18-month reactor cycle, as well as from the occasional (<1 in 10,000) fuel rod that develops a crack and releases a small portion of its volatile fission products. Uranium dioxide fuel is very resistant to erosion by the coolant, so the rod does not dump its entire fission product inventory into the RCS. 

Middle distillate fuels are blended from light gas oil streams. Since 1977, changes in the refining of middle distillate fuel, with the use of a catalytic cracking process, have lead to increased microbial contamination. These fuels now contain more aromatic compounds and are more soluble and more readily emidsifiable in water (Smith, 1991). Middle distillate fuels include diesel fuel and furnace oil (which in many cases are the same and both are called 2 oil). 

Despite the large fission product and actinide inventory of the fuel, the leakage rate from the fuel pins may be kept to a very small value by adequate design and careful quality control in manufacture. The coolant circuits of reactors which are refueled on-load are particularly easy to maintain at a low level of activity, on account of the ease with which defective elements can be removed. In addition to any small escape of fission products from cracks or pinholes in the cladding, some activity will generally arise from slight contamination of the outsides of the fuel pins with fuel while they are being loaded. 

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