Rate, overflow

The skimming units are divided into parallel channels and designed so that an oil droplet 150 g. or larger will be separated from the water. Droplet size is based on the oil droplet rise rate being equal to or greater than the overflow rate (Overflow rate = throughput t effective surface area.)... 

The two procedures primarily used for continuous nitration are the semicontinuous method developed by Bofors-Nobel Chematur of Sweden and the continuous method of Hercules Powder Co. in the United States. The latter process, which uses a multiple cascade system for nitration and a continuous wringing operation, increases safety, reduces the personnel involved, provides a substantial reduction in pollutants, and increases the uniformity of the product. The cellulose is automatically and continuously fed into the first of a series of pots at a controlled rate. It falls into the slurry of acid and nitrocellulose and is submerged immediately by a turbine-type agitator. The acid is deflvered to the pots from tanks at a rate controlled by appropriate instmmentation based on the desired acid to cellulose ratio. The slurry flows successively by gravity from the first to the last of the nitration vessels through under- and overflow weirs to ensure adequate retention time during nitration. The overflow from the last pot is fully nitrated cellulose. 

The sohd can be contacted with the solvent in a number of different ways but traditionally that part of the solvent retained by the sohd is referred to as the underflow or holdup, whereas the sohd-free solute-laden solvent separated from the sohd after extraction is called the overflow. The holdup of bound hquor plays a vital role in the estimation of separation performance. In practice both static and dynamic holdup are measured in a process study, other parameters of importance being the relationship of holdup to drainage time and percolation rate. The results of such studies permit conclusions to be drawn about the feasibihty of extraction by percolation, the holdup of different bed heights of material prepared for extraction, and the relationship between solute content of the hquor and holdup. If the percolation rate is very low (in the case of oilseeds a minimum percolation rate of 3 x 10 m/s is normally required), extraction by immersion may be more effective. Percolation rate measurements and the methods of utilizing the data have been reported (8,9) these indicate that the effect of solute concentration on holdup plays an important part in determining the solute concentration in the hquor leaving the extractor. 

The most important feature of the pressure filters which use hydrauHc pressure to drive the process is that they can generate a pressure drop across the medium of more than 1 x 10 Pa which is the theoretical limit of vacuum filters. While the use of a high pressure drop is often advantageous, lea ding to higher outputs, drier cakes, or greater clarity of the overflow, this is not necessarily the case. Eor compressible cakes, an increase in pressure drop leads to a decrease in permeabiUty of the cake and hence to a lower filtration rate relative to a given pressure drop. 

When the overflow clarity is independent of overflow rate and depends only on detention time, as in the case for high soHds removal from a flocculating suspension, the required time is deterrnined by simple laboratory testing of residual soHd concentrations in the supernatant versus detention time under the conditions of mild shear. This deterrnination is sometimes called the second-order test procedure because the flocculation process foUows a second-order reaction rate. 

The conventional one-pass clarifier is designed for the lowest specific overflow rate (flow per unit area of Hquid surface), which is usually 1—3 m/h depending on the degree of flocculation. These clarifiers can be started and stopped without difficulty. 

Flocculation is accelerated and higher overflow rates are achieved by external or internal recirculation of settled soflds into the feed which leads to the collection of fine particles by interception. Addition of conditioned fine sand to the feed induces separation by differential sedimentation, and sometimes increases overflow rates to 6—8 m/h. 

Sludge-blanket clarifiers are difficult to start up because the first blanket must be estabUshed, and large-scale units require extensive excavation. Sizes range from 600 x 600 mm to 50 x 50 m. Precipitation and crystallization can be carried out in similar hopper-designed units, having overflow rates of 80 m/h or higher. 

The settling soHds and some Hquid move downward. The amount of the latter depends on the underflow withdrawal rate. Most of the Hquid moves upward and into the overflow which is coUected in a trough around the periphery of the basin. 

Fig. 8. Sedimentation equipment performance where the particles have a A5 value of 1.0 g/cm and a viscosity, p., value of 1 mPa-s(=cP). The value of is twice the settling velocity at G = 1, and Q = overflow discharge rate in measurements given.

The diameter of the air core varies with the feed volumetric flow rate. If the rate is too low, there is no air core and all of the pulp leaves the cyclone as underflow if the rate is too high, the air core expands, closing off the apex and forcing all of the pulp to leave the cyclone as overflow. Consequently there is a minimum and maximum volumetric feed rate. Because the pressure drop is proportional to the square of the volumetric feed rate, the minimum and maximum rates can be monitored by the pressure drop. The ratio of the maximum pressure drop to the minimum pressure drop should be less than 4, meaning the maximum to minimum volumetric feed rate should be less than 2. 

In order to handle accidental spills or overflows, a spill basin maybe provided, into which the flow is diverted if the concentration of a particular constituent exceeds a predetermined value. If equalization precedes biological treatment, in addition to the organic loading, high fluctuations in temperature, salinity, and toxic organics must also be considered. After the spill is contained, the wastewater flow is diverted back to the equalization basin. The contents of the spill basin are then pumped at a constant controlled rate to the equalization basin. 

Suspended Solids Removal. Depending on the concentration and characteristics of the suspended soflds, they can be removed by filtration, flotation, or sedimentation. Coarse soflds are removed by screening. Settleable suspended soflds are removed in a clarifier, which may be circular or rectangular. The efficiency of soflds removal is a function of the overflow rate (m /-d (gal/ft -d) as shown in Figure 5. 

Fig. 5. Relationship between suspended soflds removal design parameters and overflow rate.

Silver is most commonly recovered by electrolysis or metallic replacement from the processing solutions or by ion exchange (qv) from the wash water (123). Loss of chemicals from one tank into the next has been minimized. The color paper process has progressed from five chemical solutions, three washes, and a replenishment rate of 75 lL/cm (70 mL/ft ) of film for each of the five solutions to two chemical solutions, one wash, and replenishment rates of 15 lL/cm (16 mL/ft ) and 5 lL/cm (5 mL/ft ). For color negative films, developer replenishment has dropped from over 300 to 43 lL/cm (40 mL/ft ). Regeneration of the now reduced overflow has decreased chemical discharge by as much as 55% (124). 

Multiple IJquid-Path Plates. As the Hquid flow rate increases in large diameter crossflow plates (ca 4 m or larger), the crest heads on the overflow weirs and the hydrauHc gradient of the Hquid flowing across the plate become excessive. To obtain improved overall plate performance, multiple Hquid-flow-path plates maybe used, with multiple downcomers. These designs are illustrated and discussed in detail in the Hterature (49). 

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