12 replies to this topic

[mjpotts]

I need to know how size a flash drum. I know the Flow rates of all the streams and the conditions. Is there a relatively simple equation that can be used to to calculate height and diameter or volume of the flash drum?
Thanks for all the help, mike

[mbeychok]

mjpotts:

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(or a flashing liquid) is fed and wherein the liquid is separated by gravity, falls to the bottom of the vessel, and is withdrawn.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 of a flash drum (or knock-out pot, or vapor-liquid separator) 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).

For the maximum design vapor velocity (which will set the drum's diameter), use the Souders-Brown equation:

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

where:
Vmax = maximum design vapor velocity, ft/sec
dL = liquid density, lb/ft3
dV = vapor density, lb/ft3
k = 0.35 (when the drum includes a de-entraining mesh pad)

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

0 psig:    0.35   
300 psig:  0.33
600 psig:  0.30
900 psig:  0.27
1500 psig: 0.21

GPSA Notes:
1. K = 0.35 at 100 psig; subtract 0.01 for every 100 psi above 100 psig
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 section 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.

[mjpotts]

I saw that in one of your other posts. I am a little confused what to do with that formula though. How do I get height and diameter from that?

[mbeychok]

mjpotts:

Your original posting said that you knew the flow rates of all the streams. Thus, I assume that you know the cubic feet per second of vapor (V) that will be leaving from the top of the flash vessel. Thus, the cross-sectional area (A) of the vertical flash drum will be:

A, ft^2 = (V, ft^3/sec) ÷ (Vmax, ft/sec)

Knowing the cross-sectional area A in square feet, you can then calculate the vessel diameter in feet (D).

I also assume that you know the cubic feet per minute of liquid (L) that will be leaving the bottom of the flash vessel. Multiply that by 5 to obtain volume of liquid capacity below the liquid level (i.e., 5 minutes of liquid inventory). Assuming the liquid level will be at about the midpoint of the vessel height H, then :

H, ft = (2) (5 min) (L, ft^3/min) & divide; (A, ft^2)

Now, you have the height and the diameter of the vessel. If that height to diameter ratio is about 3 to 4, you are done. If it is less than 3 to 4, increase the length of the vapor section above the liquid inventory. If it is more than 3 to 4, you may have to consider lowering the amount of liquid inventory so as to shorten the vessel or perhaps even swaging the liquid storage section to a larger diameter.

Keep in mind that the height to diameter ratio is not set in stone ... it is just a rule-of-thumb that a ratio of 3-4 provides the best dimensions from the viewpoint of the cost of the vessel.

[mjpotts]

Great thanks for the help. I was assuming that the velocity was in the exit and not across the drum. now its perfectly clear. I can tell that this is going to be good resource for my last year.

[fahood]

hey i am not really familiar with that method explained. However, lets assume you know the flow rate of the feed with the compoisition of the feed and you know the temprature of the drum with pressure, even if you don't have the Temp of the drum you can calculate it....anyhow you solve the the ratio of the vapor to the feed (V/F) from that you calculate the composition of the liquid and vapor outlet and from that there are series of equations you need to use to get the size of the drum all presented in SEPARATION ENGINEERING PROCESS BY PHILIP WANKAT....take look at that book chapter 2 and if you need any more help then let me know ....take care

[thelasik]

Considering the following, for the same service and separation conditions:

Diameter, existing: X1
Diameter, new: X2

Flow, existing: Y1
Flow, new: Y2

If I want to quickly size a drum for an increased flowrate, could I just take the ratio of the flow rates and multiply that number by the existing diameter to get a new diameter? So X2=Y2/Y1*X1

OR

Do I take the square root of the ratio and multiply it by the existing drum diameter. So X2=sqrt(Y2/Y1)*X1

Which one would you pick and why?

[joerd]

Take the square root.
Since the conditions don't change, the max. allowable velocity doesn't change. The gas velocity in a vertical separator changes with the diameter squared and linearly with volumetric flow rate:
v = Y / A
where A = pi/4 * X²
so X = sqrt[ Y / (v *pi/4) ]

[SSWBoy]

Considering the following, for the same service and separation conditions:

Diameter, existing: X1
Diameter, new: X2

Flow, existing: Y1
Flow, new: Y2

If I want to quickly size a drum for an increased flowrate, could I just take the ratio of the flow rates and multiply that number by the existing diameter to get a new diameter? So X2=Y2/Y1*X1

OR

Do I take the square root of the ratio and multiply it by the existing drum diameter. So X2=sqrt(Y2/Y1)*X1

Which one would you pick and why?

Seriously, the very fact that you are asking this question is a little worrying. By 2nd grade of secondary school I could have told you the answer.

Anyway more to the point, if your flow increases by 20% you're not going to purchase a drum 10% bigger diameter, whats the point?

You would look at ways of increasing the capacity of the current drum, i.e. installation of demisting pad or vane pack, or even looking at the service the drum is in. For compressor suction drums you have to be very strict about liquid entrainment but in other services, say for the sake of argument a flash drum is used to give a liquid + vapour feed to a column a little liquid entrainment would not be ideal, but probably acceptable.

[Linas]

Regarding M. Beychok's Post above:

I get a very low value of flash drum diameter and height (0.2' X 0.2') for my feed of 2.29 l/min. Please see the excel attachment for my calculations. Can someone please check?

Any advice or criticism is welcome.

Thanks a ton!

Attached Files

•  CHE Res.xls   31.5KB   221 downloads

[Linas]

I figured out that there was a conversion error and I arrive at the following result:

Diameter of vertical drum =2.8"
Height=2.6' for a liquid inventory time of 1.5 min.

The application is for a small R&D pilot project so the numbers are not commonly seen in industrial applications.

Attached Files

•  CHE Res.xls   31.5KB   218 downloads

[apryl]

what if there are more components that are in the vapor and in the liquid what density should i used in the density of vapor and the density of liquid? should i add all the density of the component in the vapor??

[breizh]

Unclear your query !

For the density of the vapor and the liquid you need to consider the mixtures of the components for each phase, In other words, use the mixing rules.

I 've attached a link to support your design :

http://www.chemsof.c...l_Separator.xls

Many more using the search button and liquid gas separator.

Hope this helps

Breizh