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Petroleo (Petroleum) (OP)
20 Sep 06 4:34
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
Helpful Member!(2)  25362 (Chemical)
20 Sep 06 5:50

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 Vmax to minimize liquid entrainment.

Vmax = 0.034 [(ρL - ρV) ÷ ρV)] 0.5

where:

ρL = density of liquid
ρV = density of vapor       
macmet (Materials)
20 Sep 06 10:10
I have seen gas residence time defined before simply as,

(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.
Petroleo (Petroleum) (OP)
21 Sep 06 10:42
Thank you both of you for your responses. My real problem is to calculate the oil residence time and now I found out that Stokes Law would be apropriate. Do you know if there is a typical particle size of oil in water? Oil density is 810 kg/m3 and oil viscosity is 1.4 cp at Patm. I know it must depend of many variables but the minimum range would be around 150 to 250 micras?

Thanks in advance,
petroleo
25362 (Chemical)
21 Sep 06 11:34
Petroleo, do you have water settling out of the oil ?

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

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