Scraped-wall reactors

In U.S. plants hydrofluorination is carried out in two stirred fluidized-bed reactors in series, with counterflow of solids and gases. The bed to which UO2 is fed and from which exhaust gases are discharged runs at 300°C, partially converts UO2 to UF4, and reduces the HF content of the effluent gases to around 15 percent. The bed to which anhydrous HF and the partially converted UO2 are fed runs at 500°C and converts more than 95 percent of the UO2 to UF4. To prevent caking of the fluidized beds, it has been found necessary to provide each reactor with a vertical-shaft, slow-speed stirrer to scrape the reactor walls. Production rates around 700 to 900 kg/h are obtained in 0.75-m-diameter reactors. Effluent gases are filtered to remove entrained solids, cooled to condense aqueous HF, and scrubbed to remove the last traces of HF. 

A second method involves the installation of cooling cylinders and wall scrapers (Figure 9). On large reactors this is complex and expensive. Great problems will ensue if the scraped-off material remains stuck to the scraper blades. Cleaning and dismanteling are extremely difficult. 

Theoretical estimates of the erosion of the first wall and plasma contamination due to sputtering requires a knowledge of particle and photon fluxes to the wall, as well as data on the erosion yields. Sputtering will be discussed in a later part of this chapter. Here we shall briefly summarize some of the calculations done on primary fluxes in future fusion reactors. The calculations are rather uncertain because of the poor understanding of various parameters such as divertor efficiency, refueling, neutral beam heating, plasma temperature and density profiles, including the scrape-off layer in the case of divertor operated Tokamaks. 

In the next reactor design [48] mixer arms have exactly the same dimensions and shape as the internal reactor part and in the course of the run coke residue is scraped by mixer arms from the heated reactor walls. Scraped coke falls down and is collected at the bottom of the reactor and removed with part of the reaction mixture by a suction pipe. The main process products are the gas fraction (used for heating purpose), gasoline and light gas oil and paraffin fractions. 

A batch-type reactor equipped with a screw feeder and mixer is offered by US patent [49]. A special grate is mounted inside the reactor, above the cracked melted waste plastics mixture. Scraped waste plastics which are submitted to the cracking reactor melt at the grate and fall down to the reaction mixture. As in the case of previous patent description, in the course of cracking process a mixer of a special construction scrapes coke form the reactor walls and then coke is removed from the specially shaped reactor bottom by a screw transporter. The process temperature attains level of 450° C. 

Step 7. The CPVC slurry flows to the reactor centrifuge, a spinning horizontal drum. The solids are forced against the circular drum wall and are compressed there. Liquid hydrochloric acid (waste liquor) collects in the drum and overflows through an opening at one end. and the wetcake retained on the wall, which contains 90 wt% CPVC resin and 10% hydrochloric acid, is scraped out by a large interior screw conveyor. 

Experiments with laboratory monoliths of small cross-section area can lead to biased results due to an uneven flow distribution in the channels, especially close to the reactor wall. The wash-coat of the outer broken chaimels should be scraped away, and the void between the reactor wall and the monolith should be carefully plugged. To minimize wall effects, the diameter of the monolith should be ten tunes the chaimel diameter at least. Plug flow must prevail in a packed bed of crushed catalyst. The bed length and radius should be more than 50 and 10 particle diameters respectively, the flow resistance of the bed support must be unifonn throughout its cross-section, and the particle size distribution must be as narrow as possible. Otherwise, there can be oy-passes or dead vohunes. These hydrodynamic problems are overcome in a recycle loop reactor because the same physical and chemical conditions prevail everywhere. 

Wastewater treatment plants oftentimes use vessels that combine (he (slurry) bubble column with a mechanical extractor and/or a mixer. The mechanical extractor is used to scrape heavy residue at strategically located divider walls. Such a system implies that the liquid flows across the gas flow field. Siemens proprietary Attached Growth Airlift Reactor (AGAR)-Moving Bed Bioreactor is an example of such a device. 

Polypropylene was produced in three reaction trains on a location that employed nearly 300 workers. A recycle coohng line on a reactor in Train A plngged with product slurry. This was considered a regular occurrence. Standard procedures to address this plugged hne required mechanics to work under the direction of operators to rod-out the line. Part of the cleaning procedme requires the introduction of a hydraulic ram to scrape the inner walls of the recycle line. 

The hazards associated with VCM are not only angiosarcoma of the liver. There are recent studies that indicate a higher incidence of cancer of the lung, brain, and bone marrow among workers exposed to VCM. The effect of VCM may not be evident until 15 or more years after the beginning of exposure. Particularly severe exposure used to be experienced by workers who had to climb into polymerization reactors to scrape PVC residues from the walls, agitators, and so on. 

The TS samples were scraped-off layers fijom reactor walls collected on the toroidal pumped limiter (LIM sample) and on the leading edge of the neutralizers of pumped limiter (NTR sample). 

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