Low pressure filters

Depending on the adopted configuration, from the intermediate pressure separator or the fluidized bed reactor the product is discharged to a low pressure filter, where the unreacted monomer gas is recovered. 

Automatic make-down systems are usually more consistent in ensuring a constant solids solution without gels. Variations in water pressure can however affect this, so the installation of filters is always advised before the paper machine. Since the type of gel that would be formed is soft, the pressures created in an in-line filter can force the gel through the mesh. A low-pressure filter, such as at the inlet to the stock-holding tank, is advised. 

A soln. of the startg. m. and mercuric trifluoroacetate in dry methylene chloride vigorously stirred 24 hrs. at room temp., most of the solvent evaporated at low pressure, filtered, and washed with aq. 5%-Na-carbonate -> J -diolestene-3/, 6/ -diol. Y 70%. F. e. s. G. Massiot, H.-P. Husson, and P. Potier, Synthesis 1974, 722. 

Clean generator. Blow out with low-pressure, filtered, compressed air. 

Transfer switch control—inspect components and check settings of time delays, voltage sensors, and exerciser, if applicable. Clean cabinet with low-pressure, filtered, compressed air. 

All HTGRs described in this report incorporate no pressure containment of a LWR type. In the event of a substantial failure of the primary coolant pressure boundary, the reactor building is designed to release the low activity helium coolant and serve as a low pressure filtered confinement to retain the longer term (days) limited radionuclide releases calculated for the core under the heatup conditions resulting from the depressurization. Annex XV. 

In a broad sense, hydraulic filters may be described as high pressure, medium pressure and low pressure filters. High pressure filters will operate in excess of 400 bar working pressure, with flows of 4501/min (although some operate to more than 1000 bar and for flows up to 10001/min). They can be manifold mounted and include both single and duplex housings. 

The low-pressure filter is installed on the lean-glycol circuit and is extremely useful to filter all degraded products from the glycol reboiler. 

When exhausting desiccators, a filter flask trap (see Fig. 77, 19, 2) should always be inserted between the desiccator and the pump. The vacuum should be applied gradually and should not exceed about 50 cm. of mercury for models (a), (6) and (d). These desiccators may withstand lower pressures, but it is generally considered unsafe to exhaust below this pressure unless the precaution be taken of surrounding the desiccator by a cage of fine-mesh steel wire collapse of the desiccator will then do no harm.J Models (c) and (e) may be exhausted to about 20 mm. of mercury a steel wire cage must be provided for this low pressure. 

In the precoat and body feed mode, filter aids allow appHcation of surface filtration to clarification of Hquids, ie, filtration of very dilute suspensions of less than 0.1% by volume, such as those normally treated by deep bed filters or centrifugal clarifiers. Filter aids are used in this mode with pressure filters. A precoat is first formed by passing a suspension of the filter aid through the filter. This is followed by filtration of the feed Hquid, which may have the filter aid mixed with it as body feed in order to improve the permeabiUty of the resulting cake. The proportion of the filter aid to be added as body feed is of the same order as the amount of contaminant soHds in the feed Hquid this limits the appHcation of such systems to low concentrations. Recovery and regeneration of filter aids from the cakes normally is not practiced except in a few very large installations where it might become economical. 

In apphcations where the fraction of fine particles in the soHds of the feed slurry is low, a simple and relatively cheap vacuum filter can yield cakes with moisture contents comparable to those discharged by pressure filters. Vacuum filters include the only truly continuous filters built in large sizes that can provide for washing, drying, and other process requirements. 

Despite the disadvantages, the advantages of high throughputs and low moisture contents in the filtration cakes have justified the vigorous development of continuous pressure filters. 

There are many technical problems to be considered when developing a new commercial and viable filter. However, the filtration hardware in itself is not enough, as the control of a continuous pressure filter is much more difficult than that of its equivalents in vacuum filtration the necessary development may also include an automatic, computerized control system. This moves pressure filtration from low to medium or even high technology. Disk Filters. 

The Gaudfrin disk filter is designed for only relatively low pressures of 100 kPa on average and it provides for cake washing in two stages, in two separate compartments within the same vessel. 

In the propane process, part of the propane diluent is allowed to evaporate by reducing pressure so as to chill the slurry to the desired filtration temperature, and rotary pressure filters are employed. Complex dewaxing requires no refrigeration, but depends on the formation of a soHd urea—/ -paraffin complex which is separated by filtration and then decomposed. This process is used to make low viscosity lubricants which must remain fluid at low temperatures (refrigeration, transformer, and hydraulic oils) (28). 

Both vacuum and pressure filters are used. Turbidity is more easily removed by vacuum filters, usually at 85% efficiency. Flow rates are low, ca 4 mL/(cm -min) [1 gal/(ft -min)] these filters are not practical for treating large volumes. 

Two-fluid nozzles do not operate efficiently at high capacities and consequently are not used widely on plant-size spray diyers. Their chief advantage is that they operate at relatively low pressure, the hq-uid being 0 to 400 kPa/m" pressure, while the atomizing fluid is usually no more than 700 kPa/m" pressure. The atomizing fluid may be steam or air. Two-fluid nozzles nave been employed for the dispersion of thick pastes and filter cakes not previously capable of being handled in ordinaiy atomizers [Baran, Ind. Eng. Chem., 56(10), 34-36 (1964) andTurba, Brit. Chem. Eng., 9(7), 457-460 (1964)]. 

It is advisable to start a constant-pressure filtration test, like a comparable plant operation, at a low pressure, and smoothly increase the pressure to the desired operating level. In such cases, time and filtrate-quantity data shoulci not be taken until the constant operating pressure is reahzed. The value of r calculated from the extrapolated intercept then reflec ts the resistance of both the filter medium and that part of the cake deposited during the pressure-buildup period. When only the total mass of diy cake is measured for the tot cycle time, as is usually true in vacuum leaf tests, at least three runs of different lengths should be made to permit a rehable plot of 0/V against W. If rectification of the resulting three points is dubious, additional runs should be made. 

After the reactor, a filter protects the flow controller from any catalyst dust. Caution must be applied because, if significant catalyst dust collects, results can be ruined even if the filter is at a lower temperature than the reactor. The flow controller also indicates the volumetric flow of the experiment. To operate properly, the flow controller needs a minimum 5 psig pressure at the gauge before the controller. This is important at low pressure experiments. 

The filter elements should remove particles of five microns, must be water-resistant, have a high flow rate capability with low pressure drop, possess high dirt-retention capacity, and be rupture-resistant. The clean pressure drop should not exceed five psig at 100 °F (38 °C). The elements must have a minimum collapse differential pressure of 50 psig. Pleated-paper elements are preferred—provided they meet these requirements. Usually, the pleated-paper element will yield the five psig clean drop when used in a filter that was sized to use depth-type elements. This result is due to the greater surface area of the pleated element, more than twice the area of a conventional stacked disc-type or other depth-type elements. 

Plastic packing of very low pressure drop (just greater than wood slats), transfer Coefficients about same as 2-inch Raschig rings. Most useful applications in gas cooling systems or biological trickling filters. 

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