Liquid Degassing in Vertical Vessel Sizing

[Hyen]

Dear all,

This is my first internship this summer, so I am really new in this field.

I am validating a Vertical Vessel Sizing spreadsheet. One of the contidion is Liquid Degassing, the equation used was

Diameter=7608* ((Liquid Flow/3600*Liquid Viscosity)/(Liquid Density-Vapor Density))^0.5

SI unit:
Diameter(m)
Liquid Flow (m3/h)
Liquid Viscosity (cP)
Density (kg/m3)

I couldn't find the references anywhere. Can someone point me to the right direction?

Please also let me know other info. about sizing vertical vessel. The more, the better.

Thank you very much,

 

[25362]

thread124-125547

 

[mbeychok]

FemaleChee:

A vapor-liquid separator drum is a vertical vessel into which a liquid and vapor mixture (or a flashing liquid) is fed and wherein the liquid is separated by gravity, falls to the bottom of the vessel, and is withdrawn. The vapor travels upward at a design velocity which minimizes the entrainment of any liquid droplets in the vapor as it exits the top of the vessel.

The size a vapor-liquid separator drum (or knock-out pot, or flash drum, or compressor suction drum) should be dictated by the anticipated flow rate of vapor and liquid from the drum. The following sizing methodology is based on the assumption that those flow rates are known.

Use a vertical pressure vessel with a length-to-diameter ratio of about 3 to 4, and size the vessel to provide about 5 minutes of liquid inventory between the normal liquid level and the bottom of the vessel (with the normal liquid level being at about the vessel's half-full level).

Calculate the vessel diameter by the Souders-Brown equation to determine the maximum allowable vapor velocity:

V = (k) [ (dL - dV) / dV ]^0.5

where:
V = maximum allowable vapor velocity, m/sec
dL = liquid density, kg/m³
dV = vapor density, kg/m³
k = 0.107 m/s (when the drum includes a de-entraining mesh pad)

Then A, the cross-sectional area of the drum, in m² = (vapor flow rate, in m³/s) / (vapor velocity V, in m/s)

and D, the drum diameter, in m = ( 4 A / 3.1416 )^0.5

The GPSA Engineering Data Book recommends the following k values for vertical drums with horizontal mesh pads (at the denoted operating pressures):

0 barg: 0.107 m/s
7 barg: 0.107 m/s
21 barg: 0.101 m/s
42 barg: 0.092 m/s
63 barg: 0.083 m/s
105 barg: 0.065 m/s

GPSA Notes:
1. K = 0.107 at 7 barg; subtract 0.003 for every 7 bar above 7 barg
2. For glycol or amine solutions, multiply above K values by 0.6 – 0.8.
3. Typically use one-half of the above K values for approximate sizing of vertical separators without mesh pads.
4. For compressor suction scrubbers and expander inlet separators, multiply K by 0.7 – 0.8

The drum should have a vapor outlet at the top, liquid outlet at the bottom, and feed inlet at somewhat above the half-full level. At the vapor outlet, provide a de-entraining mesh pad within the drum such that the vapor must pass through that mesh before it can leave the drum. Depending upon how much liquid flow you expect, the liquid outlet line should probably have a level control valve.

As for the mechanical design of the drum (i.e., materials of construction, wall thickness, corrosion allowance, etc.), use the same methodology as for any pressure vessel.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[Hyen]

Hi mbeychok,
Thank you for all those useful info. However, in my spreadsheet, base on different Gas Handling Capacities, preliminary diameters were calculated for each condition.

Clean Conditions
High Pressure >20 Bar g
Vacuum Operation
Highly Corrosive
all these four conditions above used V = (k) [ (dL - dV) / dV ]0.5

Liquid Degassing
Diameter=7608* ((Liquid Flow/3600*Liquid Viscosity)/(Liquid Density-Vapor Density))^0.5
SI unit:
Diameter(m)
Liquid Flow (m3/h)
Liquid Viscosity (cP)
Density (kg/m3)


My question is about the Liquid Degassing Condition. Do you know what this formula is about? and also can you point me the right reference.

Thank you again,

 

[mbeychok]

FemaleChee:

Sorry, that equation for liquid degassing is new to me. I have never seen that equation before.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[MikeClay]

The equation is based on stokes law with the assumption that the bubble rise of a 200 micron bubble is greater than the downward bulk liquid flow.

--Mike--

 

[Hyen]

thanks, MikeClay. Ima try that out.

 

[25362]

You may check the validity of the equation (which certainly includes a drag function) against air-water separation terminal velocities given in Perry VI chapter 18 for bubbles' size ranging 10 to 300 micrometers.

Another source gives measured terminal velocities of 100 mm/s for 1 mm diameter spherical bubbles, peaking at 350 mm/s for 1.8 mm dia., and dropping back to about 250 mm/s for bubbles of 3 to 4 mm dia.

Swarms of bubbles may act differently from a single bubble due to deformation of spherical shape, pulsations and bubble interactions.

A reference taken from a ChE book points out that rising velocities of bubble swarms in sea water, acetic acid, ethyl acetate, and glycerine are given by Houghton et al in Mechanism of formation of gas bubble-beds, Chem. Eng. Sci., 7, 40-50 (1957)