Sizing gravity settling method

IV.2.1 Sizing vapor/liqutd separator by gravity settling method... 

Low shear process will produce larger liquid droplets (>30 microns), and heavy/light liquids can be separated using gravity settling method. Examples of low shear process are solvent extraction, mixer settlers, steam stripping, washing process, and counter-current tower. Coalescer pad can be used to speed up the liquid/liquid separation and reduce the separator size. 

Vessel sized for liquid surge drum and vapor/liquid separation by gravity settling method can be used for final design. Vessel siz for vapor/liquid separation by mist eliminator method should be checked by the mist eliminator vendor to confirm its size. Vessel sized for Hquid/liquid separation by surge volume method or with coalescer pad is a rough preliminary sizing. Its size should be checked by vendor or an expert. 

The measured diameters of particles shoiild as nearly as possible represent the effective particle size of a dust as it exists in the gas stream. When significant flocculation exists, it is sometimes possible to use measurement methods based on gravity settling. 

In gravity settling the sample is dispersed in a liquid and then allowed to settle in a sedimentation cell. The height of the particles in the cell is then measured, or the specific gravity of the dispersing liquid is measured at different time intervals, which gives an indication of the size distribution. The hydrometer, or Bouyoucos, method is frequently used to classify the amount of sand, silt, and clay in soils. 

In sedimentation methods, dependent on gravity, particle size is determined from settling velocity and under size fraction is determined by changes in concentration in a settling suspension (Snow et al., 1999). Stokes Law relates particle size to settling velocity ... 

MF may be used to remove these heavy metals provided pretreatment chemicals are added to precipitate the metals to particles of filterable size. The chemical pretreatment step is crucial since it will affect the performance of the membrane and the resultant sludge volume as well as the contaminant removal efficiency. Reduction/oxidation, absorption/oxidation, and/or catalytic reactions are utilized along with pH adjustment to provide the optimum precipitation. Although conventional methods of waste water treatment may use a similar pretreatment chemistry, the final solid/liquid separation by gravity settling is usually not as effective as membrane filtration. 

The underlying principle behind the gravity sedimentation method is Stokes law, which describes the relationship between the settling velocity of particles in a fluid medium of known density and viscosity [6], A number of instruments are available in which the settling velocity of particles is measured by x-ray absorption, light absorption, and density changes. One such instrument is the Sedigraph by the Micromeritics Corporation in which the particle size distribution is determined by x-ray absorption [6]. This instrument offers an excel-... 

From these- equations one may calculate the time necessary for a particle of a given size to travel a cenain distance in a fluid. Spherical particles of 1 im diameter require, for example, 3.09 hours in water, or 2.08 minutes in air to travel a distance of one centimeter. A reduction of particle size to 0.1 im decreases the settling rate by a factor of 100. Even if one could neglect other factors, such as Brownian motion, convection currents and the like, it would therefore not be advisable to use gravity sedimentation for the determination of small panicles. No wonder that gravity sedimentation methods have almost disappeared from the market. Even if the height of fall is drastically reduced, the time for one analysis is too long to be accepted nowadays in the routine analysis of particle size distributions. 

Elutriation differs from sedimentation in that fluid moves vertically upwards and thereby carries with it all particles whose settling velocity by gravity is less than the fluid velocity. In practice, complications are introduced by such factors as the non-uniformity of the fluid velocity across a section of an elutriating tube, the influence of the walls of the tube, and the effect of eddies in the flow. In consequence, any assumption that the separated particle size corresponds to the mean velocity of fluid flow is only approximately true it also requires an infinite time to effect complete separation. This method is predicated on the assumption that Stokes law relating the free-falling velocity of a spherical particle to its density and diameter, and to the density and viscosity of the medium is valid... 

Sedimentation techniques such as gravity and centrifugal settling are fairly simple methods of determining the size of particles in soils and in matrices, such as paints and ceramics, where screening is not practical. For some analysis, particularly of soils, the time required to perform the analysis may take as long as 24 h. 

In actual applications, the gas flow in a gravity settler is often nonuniform and turbulent the particles are polydispersed and the flow is beyond the Stokes regime. In this case, the particle settling behavior and hence the collection efficiency can be described by using the basic equations introduced in Chapter 5, which need to be solved numerically. One common approach is to use the Eulerian method to represent the gas flow and the Lagrangian method to characterize the particle trajectories. The random variations in the gas velocity due to turbulent fluctuations and the initial entering locations and sizes of the particles can be accounted for by using the Monte Carlo simulation. Examples of this approach were provided by Theodore and Buonicore (1976). 

---