Pressure vessel aerators

In contrast to vacuum flotation, dissolved-air flotation units can be operated on a continuous basis by the application of pressure. This consists of pressurizing and aerating the process stream and introducing it into the flotation vessel that is maintained at the atmospheric pressure. The reduction of pressure results in the formation of fine air bubbles and the collection of fine particulates to be floated and removed as sludge. 

Charge 2.6 g of fentanyl citrate into a pressure addition vessel and dissolve with stirring in 405.6 g of ethanol in which 0.26 g of oleic acid has previously been dissolved. After sealing and evacuation thereof, 6.7 kg of HFA 134a, which has previously been aerated with carbon dioxide... 

Charge 112.5 g of micronized lomustine into a pressure-addition vessel. After sealing and evacuation thereof, 10.5 kg of HFA 227 that has been aerated with carbon dioxide and adjusted to a pressure of 4.5 bar (20°C) in another pressure addition vessel in which 312 g of ethanol has... 

Circulation models indicate N6 to be also dependent upon the heights of the impeller from the bottom of the vessel and the heights of the unagitated liquid level (see Eqs. (2.11)—(2.14). For an aerated vessel, the use of Eq. (2.15) for NO is recommended. Generally, both gas and liquid are considered to be completely backmixed for a stirrer speed greater than N0. While more information on gas-phase backmixing is needed, if the reaction depends on the partial pressure of gas, an optimum impeller speed to achieve the maximum space-time yield lies in the vicinity of critical impeller speed. 

Pressurization could be carried out on the entire feed stream (full-flow pressure flotation) or a fraction of the feed stream while the remainder is introduced directly without aeration into the flotation tank (split-flow pressure flotation). The spht-flow system offers a cost saving over the full-flow units, since only a portion of the influent needs to be pressurized. In both cases, however, if the sohd particles in the feed stream are flocculated before introducing to the flotation tank, the high shear during pressurization, aeration, and pressure release can destroy the floes. Also, if the particle loading in the feed stream is high, both systems are susceptible to block e of the air release devices. To minimize these problems, recycle-flow pressure flotation is often practiced (Fig. 19-71). In this process, the feed stream, flocculated or otherwise, is introduced directly into the process vessel, and part of the clarified effluent is pressurized, aerated, and recycled to the flotation tank in which it is mixed with the flocculated feed. The air bubbles are released as they attach to the floes and float to the tank surface. The recycle-flow devices are found to offer the highest unit capacities. 

To achieve a proper de-aeration, the bleached oil is sprayed into a vessel under reduced pressure, before entering the heating section. The lower the pressure applied, the lower the residual oxygen level in the oil. Usually, the oil is heated to at least 80° C and sprayed in a tank, which is kept at a pressure below 50 mbar. Some refiners even use the low pressure of the deodorizer or add some sparge steam in the spraying vessel to improve de-aeration. 

Producing low pressures in the tank also accelerates de-aeration but the foam formed on the interface may be quite stable. Usually large tanks used for mixing liquids are not built to withstand vacuum. There are commercially available continuous de-aerators based on the formation of a film on a vessel wall and subjecting it to vacuum and/or centrifugal force. They tend to fail when the foam produced is well stabilized. 

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