Scrub circuit

Howden An early flue-gas desulfurization process using a lime or chalk slurry in wooden grid-packed towers. The calcium sulfate/sulfite waste product was intended for use in cement manufacture, but this was never commercialized. The key to the process was the use of a large excess of calcium sulfate in suspension in the scrubbing circuit, which minimized the deposition of scale on the equipment. The process was developed by Imperial Chemical Industries and James Howden Company in the 1930s and operated for several years at power stations at Fulham, London, and Tir John, South Wales, being finally abandoned during World War II. British Patents 420,539 433,039. 

In the gas cleaning sections of spent acid or metalluigical sulfuric acid plants, the weak acid scrubbing circuit is typically handled by plastic or glass fiber reinforced plastic (FRP) pipe. The contaminants in weak acid usually vary too greatly to allow use of an economical alloy. 

To prevent accumulation of ash and slag in the water circuit, some water is discharged continuously. In a combined chemical and physical treatment sulfides, cyanides and suspended solids (ash) are removed. A concentrated sludge has to be filtered off and disposed of. The run-off water from the filters is stripped of ammonia and after pH-adjustment sent to a biological treatment unit. Uhde [532] has developed an alternative route for soot treatment in which the soot is filtered off and subjected to combustion, and the filtered water is recycled to the quench and scrubbing circuits. Let down water is treated as described above. This soot treatment technique avoids not... 

The ammonium salt products crystallized from the concentrated spent scrubber liquors may be used as valuable constituents of fertilizers. Or they may be first reduced to ammonia and hydrogen sulfide with natural gas, followed by conversion of the hydrogen sulfide to sulfur by a Claus-type sequence [39]. In this French-designed modification, the ammonia is recycled to the scrubbing circuit. 

In some flow sheets, a scrub circuit is introduced between the extraction and stripping circuits. A scrub liquor is introduced to scrub off unwanted coextracted species from the loaded organic phase by displacing the impurities with the more strongly complexed main element. This produces a scrubbed organic phase. For systems in which the extractant is completely selective for the species of interest or when high selectivity of extraction is not required (such as for bulk extractions), a scrub circuit is not necessary. Usually, however, some coextraction of other species occurs and a scrub circuit is anployed to reduce the transfer of these species to the LSL. 

AEA also concluded that the agent offgas/scrubbing systems could be fabricated from stainless steel, while the agent anolyte and catholyte circuits should be lined with materials resistant to corrosion by free fluoride, e.g., PTFE or PFA. 

In some scrub and strip circuits, the crud is mainly composed of silica, as well as inorganic sulfates. Also, if poor pH control is used in the uranium stripping circuits with ammonium sulfate, then uranium is a major constituent [33,46]. Such crud may be treated with dilute sulfuric acid, and recirculation through a pump results in the crud breaking down. There is evidence, in at least a few uranium circuits, that the presence of humic acids may be a possible cause of the crud problem [34,47]. Lignin appears to be another cause of crud formation [33,46]. Humic acids contained in the feed solution have also been implicated in the formation of waxy cruds in plants extracting uranium from phosphoric acid. 

Because crud is a difficult phenomenon to completely characterize, is often site-specific, and indeed can vary within an extraction, scrub, and strip-circuit operation, preventative measures are difficult to cite. The following are some of the methods that can be suggested for the prevention of crud. Again, it must be emphasized that because cruds have a different history of formation, one or more preventive measures may be necessary. 

Occasionally, the water used for solution makeup to the scrub or strip circuits, because of impurities, can cause subsequent emulsions and cruds. At one uranium plant in South Africa, deionized water was used to prepare the ammoniacal strip solution, rather than normal plant water, which tended to cause crud formation [53,54]. 

In the absorber, the formaldehyde is absorbed in water or urea solution. Heat is removed by one or two cooling circuits (8, 9). From the lower circuit (8), product in the form of either AF or UFC is withdrawn. Scrubbed gas from the absorber is split in two streams—recycle gas and tail gas. The tail gas is vented after any organic impurities are catalytically incinerated in the reactor (10). Thus, the tail-gas purity conforms to the environmental standards for any country. 

In the new process, other separation zones are added after the normal first separation zone. The heavy phase from the first zone is sent to a suipper and the vaporized solvent, steam, and fine mist particles are withdrawn from the top and are conveyed to the second separation zone. In the second separation zone, the stream consisting of solvent, steam, and mist particles flow through a circuitous path where it is contacted couniercurrently with a sU eam of light oil which scrubs the asphaltene and resin mist from the solvent. A portion of the light phase oil containing asphaltenes and resins is recycled to the second separation zone or it can be recycled to the first separation zone. The solvent and steam mixture leaving the second zone is conveyed to a solvent condenser and water separation zone. No emulsification problems are exp>erienced because the mist particles have been removed in the first separation zone. 

Control Cell The cell contains a posting-in facility from the decontamination cell, an in-cell crane, a smear test machine to externally scrub the container, a pneumatic scrub transfer machine to allow swab monitoring, and a gamma gate for flasking out containers to the product store. The function of the cell is to monitor the external surface of containers prior to discharge to the product store viewing is via closed circuit television. 

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