Droplet settling velocity

Most studies have used the Souders-Brown [67] droplet settling velocity concept to relate entrainment flooding. In this mechanism, flooding develops due to a sufficiently high upward vapor velocity through the cross-section of the net area of the column to suspend droplets, and is expressed as the Souders-Brown flooding constant, Csb> [94, 183, 184]. 

The motion of aerosol droplets following space spraying is governed by gravity and the resistance of air particle motion. Droplet settlement depends on the size of the droplet. The droplet settling velocity (Vi) (m/s) is given by Stokes law (Hinds, 1982) ... 

For those applications where a more complete design is required, the starting point for the entrainment reduction check is the droplet settling velocity. 

Once drag coefficient (C) is known, liquid droplet settling velocity (vs) can be calculated using Eq. (3a), and vessel size can be calcul ed as follows. 

For horizontal separator, liquid droplet settling velocity (vs) is still calculated by Eq, (3a). But for horizontal separator, liquid droplets are settled in vertical direction perpendicular to vapor flow velocity (v, in ft/sec). For horizontal separator, there is a requirement for its vessel length (L). The requirement is that the vessel should be long enough for the selecled minimum size liquid droplets to settle into the liquid phase before the vapor flow exit the vessel. Mathematically, following equation should be valid ... 

Rcanange Eq. (4d) to show the relationship of vapor velocity (v) and liquid droplet settling velocity (vs) as... 

A2.7.2 Interfacial tension values may be significantly affected by additives and contaminants. If it is known that the value is other than 0.025 N/m, the water droplet settling velocity W, given in A2.8, should be modified by multiplying by Eq 9. 

The metiiod presented in API 521 is mostly used in the sizing of KO drums. This method is based on the droplet settling velocity and does not calculate the K factor velocity. 

As shown by Fig. 14-90, entrainment droplet sizes span a broad range. The reason for the much larger drop sizes of the upper curve is the short disengaging space. For this cui ve, over 99 percent of the entrainment has a terminal velocity greater than the vapor velocity. For contrast, in the lower cui ve the terminal velocity of the largest particle reported is the same as the vapor velocity. For the settling velocity to limit the maximum drop size entrained, at least 0.8 m (30 in) disengaging space is usually required. Note that even for the lower cui ve, less than 10 percent of the entrainment is in drops of less than... 

Gravity Settlers Gravity can act to remove larger droplets. Set-thng or disengaging space above aerated or boiling liquids in a tank or spray zone in a tower can be very useful. If gas velocity is kept low, all particles with terminal settling velocities (see Sec. 6) above the gas... 

Note that the absolute value of Pp - p has been assumed. The negative value of this difference indicates that the droplet displacement is centripetal. The value of the Reynolds number corresponding to Ar = 3650 from Figure 10 is 45. Hence we can determine the radial settling velocity from the definition of the Reynolds number ... 

We have an emulsion of oil in water that we need to separate. The oil droplets have a mean diameter of lO " m, and the specific gravity Of the oil is 0.91. Applying a sedimentation centrifuge to effect the separation at a spedd of 5,000 rpm, and assuming that the distance of a droplet to the axis of rotation is 0.1 m, determine the droplet s radial settling velocity. 

The top layer will be hydrocarbon, with the aqueous layer droplets settling through the hydrocarbon. The terminal velocity is ... 

Vgq = terminal settling velocity of hydrocarbon droplets in aqueous phase in bottom of vessel, in./min flaq viscosity of aqueous phase, cp... 

The terminal (highest calculated) settling velocity of the aqueous droplet in/through the hydrocarbon phase is ... 

The decanter vessel is sized on the basis that the velocity of the continuous phase must be less than settling velocity of the droplets of the dispersed phase. Plug flow is assumed, and the velocity of the continuous phase calculated using the area of the interface ... 

Equation 10.7 is used to calculate the settling velocity with an assumed droplet size of 150 pm, which is well below the droplet sizes normally found in decanter feeds. If the calculated settling velocity is greater than 4 x 10-3 m/s, then a figure of 4 x 10 3 m/s is used. 

Equation 10.10 can be used to estimate the settling velocity of the liquid droplets, for the design of separating vessels. 

The terminal settling velocity is given by Equation 8.6 or 8.8. Decanters are normally designed for a droplet size of 150 p,m3,9, but can be designed for droplets down to 100 p,m. Dispersions of droplets smaller than 20 p,m tend to be very stable. The band of droplets that collect at the interface before coalescing should not extend to the bottom of the vessel. A minimum of 10% of the decanter height is normally taken for this3. 

For preliminary design, liquid entrainment is usually used as a reference. To prevent entrainment, the vapor velocity for tray columns is usually in the range 1.5 to 3.5 ms-1. However, the entrainment of liquid droplets can be predicted using Equation 8.3 to calculate the settling velocity. To apply Equation 8.3 requires the parameter KT to be specified. For distillation using tray columns, KT is correlated in terms of a liquid-vapor flow parameter FLV, defined by ... 

Vapor-Liquid Gravity Separator Design Fundamentals The critical factors in the performance of a horizontal separator are the vapor residence time and the settling rate of the liquid droplets. However, two other factors enter into the design—the vapor velocity must be limited to avoid liquid entrainment, and there must be sufficient freeboard within the vessel to allow for a feed distributor. For vertical separators, the design is based on a vapor velocity that must be less than the settling velocity of the smallest droplet that is to be collected, with due allowance for turbulence and maldistribution of the feed. The vapor residence time is a function of the vapor flow rate (mass), vapor density, and volume of vapor space in the separator, based on the following ... 

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