Sedimentation vessel

It corresponds to the amount of surface that would be required in a sedimentation vessel to achieve the same results as in a centrifugal separator (such as the Westfaila). It can be used for comparing the capacities of separators of different size of design. Specifically, the capacities gi and Q2 of two separator are related as follows ... 

In the Sartorius balance [66-68] the pan is suspended in the suspending liquid and a correction has to be applied for the particles which fall between the rim of the pan and the sedimentation vessel. In this instrument, when 2 mg of sediment has deposited, electronic circuitry activates a step-by-step motor which twists a torsion wire to bring the beam back to its original position. A pen records each step on a chart. The manufacturers suggest that about 8% of the powder does not settle on the pan. Leschonski [69] and Leschonski and Alex [70] reported losses of between 10% and 35%, depending on the fineness of the powder the difference was attributed to the pumping action of the pan as it re-balances. 

Fig. 1. Schematic diagram of the apparatus. 1 — column I, 2 — column II, 3 — column III, 4 — column IV, 5 — sedimentation vessels, 6 — pumps, 7 — sintered glass plates,...

The sensing-zone techniques are not limited to conductivity but may involve the measurement of capacitance. Figure 2 shows an example of the use of capacitance monitoring in a vertical sedimentation vessel. In this case (14) the effective suspension permittivity was measured and used to estimate the solids concentration (expressed in terms of volume fraction). 

Figure 2. Capacitance monitoring in a sedimentation vessel (inset) and the relation between effective suspension permittivity ee and solids concentration in the suspension for spherical particles of permittivity es in liquid of permittivity eL. (Reproduced from reference 14. Copyright 1992 Butterworth-Heinemann.)...

The equivalent spherical particle diameter of an aggregate of irregularly shaped particles can be found by studying the inertial motion of particles in a medium. This inertial motion behavior is used in many applications, such as sedimentation vessels, electrostatic separators and precipitators, and particle collectors. The various forces that affect particle motion, shown in Figure 4, are briefly discussed below. 

Gravitational Sedimentation. There are four main gravitational sedimentation techniques (1) sedimentation vessel wall pressure sensing, (2) manometry, (3) volume sample, and (4) mass sample, as shown schematically in Figure 31 (S, 18, 35). 

Zone sedimentation of aggregating suspensions coincides with the formation of a particle network (gel) in the lower part of the sedimentation vessel. This gel is gradually consolidated under its own weight, which reduces its porosity and the sedimentation velocity. The gel is, therefore, called compression or compaction zone, it eventually comprises aU particles of the suspension. The compression zone has a much higher porosity than the sediment of a weU-stabrhsed suspension (Sobisch et al. 2006). For fast aggregation and very fine particles, the gelation can finish even before a visible sedimentation of the dispersed phase occurs. 

Most sedimentation vessels are engineered for continuous rather than batch flow. The general design goal for a continuous sedimentation vessel is to engineer quiescent conditions such that the downward settling velocity exceeds the horizontal velocity of the particles across and out of the vessel. The trajectory of a particle in a sedimentation vessel is determined by its horizontal and vertical velocity components. In concept, a particle drops the same distance in a circular vessel as it does in a rectangular reactor that has an equivalent characteristic time (X = V70, or in a batch reactor after a duration t = X (Benjamin and Lawler, 2013). 

Sedimentation vessels are typically either rectangular or circular in shape, and provide for feed of the wastewater, removal of a solids concentrate, and removal of a clarified effluent. It is also common to recycle a proportion of the solids concentrate back into the feed side of the vessel in order to help seed precipitation reactions and aid in the settling. A rectangular vessel allows for efficient use of space and facilitates an efficient inlet design that provides for even spreading of the influent. A circular vessel makes for less efficient use of space and a more complicated inlet design, but is typically designed with a conical bottom and scraper that facilitates removal of the solids concentrate from the bottom. The wastewater in a circular sedimentation basin is introduced into a center well and removed via a weir system at the periphery, such that the particle trajectory is radially outward. 

All particles have the same density p and any influences from the wall of the sedimentation vessel are negligible. 

A centrifuge can offer an alternative accelerated process via the increased force of gravity. A solid bowl centrifuge operates as the sedimentation vessel and there are numerous... 

The alternative experimental procedure of withdrawing the hydrometer between readings over a period of time gives rise to two types of error. When the hydrometer is inserted into the suspension, it displaces and generally disturbs the suspension as it sinks through it. The effect of this disturbance is difficult to estimate, and the cumulative result of a series of insertions and withdrawals can be measured only empirically. Factors that influence the magnitude of the error caused by this disturbance of the suspension are 1) the ratios of the bulb dimensions to those of the sedimentation vessel and 2) the depth to which the hydrometer sinks. 

The upper part of the figure shows the principle of the incremental sedimentation analysis The disperse solid shall be homogenously distributed in a sedimentation vessel at the time t = 0. A sensor is situated at the depth hj, which measures the solid concentration there as a function of time. In the ideal case, the highest particle layer of a monodisperse system is just reaching the measuring plan at the time t = tj. This leads to the concentration jump at this point, shown below. Consequently, the resulting cumulative distribution, represents monodispersity. 

The solution of the Fick s equation for a sedimentation vessel with a suspension level and with a bottom has been found by Meason et al. / / and later by Furth 111 nearly 70 years ago. Their equations are complicated and very difficult to treat, and therefore frequently you find in the literature /3, 4, 5/ the more simple solution for an infinite long vessel. 

Concentration distribution in the sedimentation vessel at the left exact values, at the right approximation... 

There the relation is shdwn between the relative solid concentration and the sedimentation path for quartz particles in water. The measuring plane is in the middle of the vessel. After the sedimentation time for a particle of 0.1 pm size has passed, concentration distributions are found, which only near the measuring plane and also near the suspension level are described with sufficient accuracy by the approximation equation. As shown in /6/, the latter, however, is only applicable, if the particles are greater than 0.1 pm. If only the mechanisms in the middle of a sufficiently big sedimentation vessel are considered, then the theory gives the following expression for the measured apparent distributions of monodisperse particles ... 

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