Decanters

Decanters are used for the separation of two immiscible liquids of different densities from one another. This process is called decantation.

The difference in densities of two immiscible liquids is responsible for such a separation.

Separation of two immiscible liquids based on the density difference of the phases involved is commonly encountered in the mass transfer operation such as liquid liquid extraction.

Types of Decanters

There are mainly two types of decanters used for the separation of two immiscible liquids.

Gravity Decanter
Centrifugal Decanter

Continuous Gravity Decanter

A gravity decanter is used for the separation of two immiscible liquids when the difference between the densities of the two liquids is large.

The feed mixture consists of two liquids entering at one end of the separator. The two liquids flow slowly through the vessel, separate into two layers, and discharge through overflow lines at the other end of the separator.

The overflow lines provided in the decanter are so large, therefore frictional resistance to the flow of the liquids is negligible. Liquids discharge at the same pressure as that in the gas space above the liquid in the vessel. The performance of the decanter can be analyzed by the principles of fluid statics.

For the decanter shown in Figure.
Let the density of the heavy liquid be ρ_A and the density of the light liquid be ρ_B.
The depth of the layer of heavy liquid is Z_A1 and that of the light liquid is Z_B.
The total depth of liquid in the vessel Z_T is fixed by the position of the overflow line for the light liquid.
Heavy liquid discharges through an overflow leg connected to the bottom of the vessel and rises to a height Z_A2 above the vessel floor.
The overflow lines and the top of the vessel are all vented to the atmosphere.

Since there is negligible frictional resistance to flow in the discharge lines, the column of heavy liquid in the heavy-liquid overflow leg must balance the somewhat greater depth of the two liquids in the vessel.

A hydrostatic balance leads to the equation

$Z_B \;\rho_B + Z_{A1} \; \rho_B = Z_{A2} \; \rho_A \; ----- (1)$

Solving equation (1) for $Z_{A1}$

$Z_{A1} = Z_{A2} - Z_B \frac {\rho_B}{\rho_A}$

Here, $Z_T = Z_B + Z_{A1}$

$Z_{A1} = Z_{A2} - (Z_T - Z_{A1}) \frac {\rho_B}{\rho_A}$

$Z_{A1} = \frac {Z_{A2} - Z_T (\frac{\rho_B}{\rho_A})}{1 - \frac{\rho_B}{\rho_A}} ------ (2)$

Equation (2) shows that the position of the liquid-liquid interface in the separator depends on the ratio of the densities of the two liquids and the elevations of the overflow lines. It is independent of the rates of flow of the liquids. Equation (2) shows that as $\rho_A$ approaches $\rho_B$ the position of the interface becomes very sensitive to changes in $Z_{A2}$ , the height of the heavy-liquid leg. With liquids that differ widely in density, this height is not critical, but with liquids of nearly the same density, it must be set with care. Often the top of the leg is made movable so that in service it can be adjusted to give the best separation.

The size of a decanter is established by the time required for separation, which in turn depends on the difference between the densities of the two liquids and on the viscosity of the continuous phase. Provided the liquids are clean and do not form emulsions, the separation time may be estimated from the empirical equation.

$t = \frac {100 \mu}{\rho_A - \rho_B} ----- (3)$

Where, t = separation time (hours), $\rho_A, \rho_B$ = densities of liquids A and B, kg/m³, $\mu$ = viscosity of the continuous phase (cP).

Equation (3) is not dimensionless, and the indicated units must be used.

Successful operation of a decanter depends on both the sedimentation and the coalescence of the dispersed phase. Equation (3) gives poor results if the liquids to be separated are not clean but contain particulates or polymeric films that reduce the rate of coalescence. Such contaminants may also lead to the formation of a dirty layer of uncoalesced droplets-called a “rag”-at the liquid-liquid interface. Coalescing devices such as beds of porous solids, membranes, or high-voltage fields are often necessary for a satisfactory separation. With cleaner liquids, the size of a decanter can often be greatly reduced by putting in horizontal or slightly inclined pipes or flat plates, so that the heavy-phase droplets need to fall only a short distance before reaching a layer of the heavy liquid.

Centrifugal Decanter

When the difference between the densities of the two liquids is small, the force of gravity may be too weak to separate the liquids in a reasonable time. The separation may then be accomplished in a liquid-liquid centrifuge, shown diagrammatically in Figure below.

It consists of a cylindrical metal bowl, usually mounted vertically, that rotates about its axis at high speed. In Figure (a) the bowl is at rest and contains a quantity of two immiscible liquids of differing densities. The heavy liquid forms a layer on the floor of the bowl beneath a layer of light liquid. If the bowl is now rotated, as in Figure (b), the heavy liquid forms a layer, denoted as zone A in the figure, next to the inside wall of the bowl. A layer of light liquid, denoted as zone B, forms inside the layer of heavy liquid. A cylindrical interface of radius r_i separates the two layers. Since the force of gravity can be neglected in comparison with the much greater centrifugal force, this interface is vertical. It is called the neutral zone.

In the operation of the machine, the feed is admitted continuously near the bottom of the bowl. Light liquid discharges at point 2 through ports near the axis of the bowl; heavy liquid passes under a ring, inward toward the axis of rotation, and discharges over a dam at point 1. If there is negligible frictional resistance to the flow of the liquids as they leave the bowl, the position of the liquid-liquid interface is established by a hydrostatic balance and the relative “heights” (radial distances from the axis) of the overflow ports at 1 and 2.

Assume that the heavy liquid, of density $\rho_A$ overflows the dam at radius r_A, and the light liquid, of density $\rho_B$ , leaves through ports at radius r_B. Then if both liquids rotate with the bowl and friction is negligible, the pressure difference in the light liquid between r_B and r_i must equal that in the heavy liquid between r_A and r_i. The principle is the same as in a continuous gravity decanter.

$p_i - p_B = p_i - p_A ----- (4)$

Where p_i = pressure at the liquid-liquid interface, P_B = pressure at the free surface of the light liquid at rB, PA =pressure at the free surface of the heavy liquid at r_A

We know,

$p_i - p_B = \frac {\omega^2 \rho_B (r_i^2 - r_B^2)}{2}$

$p_i - p_A = \frac {\omega^2 \rho_A (r_i^2 - r_A^2)}{2}$

Equating these pressure drops and simplifying leads to

$\rho_B (r_i^2 - r_B^2) = \rho_A (r_i^2 - r_A^2)$

Solving for r_i gives

$r_i = \sqrt{\frac{r_A^2 - (\frac{\rho_B}{\rho_A}) r_B^2}{1 - \frac{\rho_B}{\rho_A}}} ----- (5)$

Equation (5) is analogous to Eq. (2) for a gravity settling tank. It shows that r_i the radius of the neutral zone, is sensitive to the density ratio, especially when the ratio is nearly unity. If the densities of the fluids are too nearly alike, the neutral zone may be unstable even if the speed of rotation is sufficient to separate the liquids quickly. The difference between $\rho_A$ and $\rho_B$ should not be less than approximately 3 percent for stable operation.

Equation (5) also shows that if r_B is held constant and r_A, the radius of the discharge lip for the heavier liquid, increases, and the neutral zone is shifted toward the wall of the bowl. If r_A is decreased, the zone is shifted toward the axis. An increase in rB, at constant rA, also shifts the neutral zone toward the axis, and a decrease in rB causes a shift toward the wall. The position of the neutral zone is important practically. In zone A, the lighter liquid is being removed from a mass of heavier liquid, and in zone B, the heavy liquid is being stripped from a mass of light liquid. If one of the processes is more difficult than the other, more time should be provided for the more difficult step. For example, if the separation in zone B is more difficult than that in zone A, zone B should be large and zone A small. This is accomplished by moving the neutral zone toward the wall by increasing r_A or decreasing r_B. To obtain a larger time factor in zone A, the opposite adjustments would be made. Many centrifugal separators are so constructed that either r_A or r_B can be varied to control the position of the neutral zone.

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