## Residence time calculation

## Residence time calculation

(OP)

Hello All,

I suppose this is a so simple question but I am afraid I do not know how to solve it. Could you tell me How I can calculate if the residence time in an existinting horizontal separator is OK? I have the dimensions and estimated liquid and gas flows.

Thanks in advance,

Petroleo

I suppose this is a so simple question but I am afraid I do not know how to solve it. Could you tell me How I can calculate if the residence time in an existinting horizontal separator is OK? I have the dimensions and estimated liquid and gas flows.

Thanks in advance,

Petroleo

## RE: Residence time calculation

Since you do not provide details on the service, a general response would be:

a. The residence time of the liquid is the ratio of occupied vessel volume, at the required level, divided by the liquid flow rate. Liquid hold-up depends on the service, for example, whether it is to be used as a reflux drum, or if it is intended to allow separation of water from hydrocarbons, etc.

b. For efficient V/L separation, it is the volume above the liquid that interests us. As a ROT, the cross-section above the liquid, devoted to V-L separation is estimated so as to ensure that the horizontal component of vapor velocity, V, fps, is below a V

_{max}to minimize liquid entrainment._{max}= 0.034 [(ρ_{L}- ρ_{V}) ÷ ρ_{V})]^{0.5}where:

ρ

_{L}= density of liquidρ

_{V}= density of vapor## RE: Residence time calculation

(volume of component (ft3))/(gas flow(ft3/s))

However, this simple rule cannot be used for all applications. You will have to decide if this fits your application.

## RE: Residence time calculation

Thanks in advance,

petroleo

## RE: Residence time calculation

If so, please consider the following:

a. Stokes'law is used to determine terminal velocities of falling drops but is usually of little value since, as you say, the particle size distribution is rarely known.

b. Particles in concentrations above 0.1% by volume give reduced (hindered) settling velocities due to increases in the "apparent" viscosity and density.

c. Terminal velocity estimates do not apply to liquids that tend to emulsify. For example if sodium naphthenates or other surfactants that reduce the surface tension of water are present. Or if the oil-water mixture entering the drum previously passed through a centrifugal pump or a throttling valve.

d. ROT for determining the rate of settling of water in HCs from butane to diesel, are:

→ 1.7 mm/s or less for practically complete settling

→ 2.5-4.2 mm/s for reasonably good settling

It is generally recommended to carry out lab tests on settling rates.

Visit:

www.clarkson.edu/~wilcox/Design/coalesc.pdf