5 replies to this topic

[Chane189]

Hey!

 

We are currently designing a trayed distillation column for our final year design project. The column is a de-butaniser. We are using 2-pass valve trays for the column due to high liquid loads.

 

At the moment we are experiencing downcomer backup, jet flooding etc with the calculated downcomer area. I have downloaded KG-Tower (the tower design program from Koch-Glitsch) to assist me. Our calculated downcomer area to tower area is 13% - versus the program calculated value of 44%. With the larger downcomer area, all problems are fixed.

 

I have read that the downcomer area should be approximately 10% of the tower area. I am concerned about the smaller active bubbling area which the increased downcomer area will result in.

 

Anyone out there who can give me some advice?

 

Thanks

[Bobby Strain]

Something doesn't sound right. Maybe you should tell us more and maybe we can help.

 

Bobby

[katmar]

The 10% value is a rule of thumb that should be totally ignored.  I have replaced columns where the original downcomers were designed on the 10% rule and in the new column (with the same total tray area) the capacity increased by 50% just by correctly sizing the downcomers.

 

The important factors in the downcomers are the residence time and the back-up.  The area that you determine is calculated to satisfy these 2 criteria.  So the area is a consequence of other decisions, rather than an independent variable that you set.

 

44% is on the high side of the range of areas that you will see, and as Bobby Strain has advised it is probably worth checking that your liquid to gas ratio is correct.  But you are already aware that the liquid load is high, so it sounds like you know what you are doing in that regard.

[Padmakar Katre]

Hi,

The issues you have mentioned such as Jet Flood and DNCR Backup are strong function of Active or bubbling area. I will recommend to follow,

1. Get the Perry's ChE handbook and Distillation Design by H.Z Kister issued from Library

2. Estimate the Column C-factor and Down-comer inlet velocity based on column area distributed between active area & DNCR area.

3. There are correlations available in the above books for maximum DNCR allowable inlet velocity which will help you in deciding the down-comer area

4. There is one parameter called System C-factor (ultimate capacity : significance is the C-factor beyond which the liquid entrainment is so sever that by providing an tray spacing you can get the liquid separation from up flowing vapors)

5. Compare the step 3 & 4 parameters with step 2. Have some margin such as step 2 parameters to be maximum of 80% of step 3&4. This will help in estimation of DNCR area and Active area hence you can calculate the column area and thereby column diameter.

 

Please note that step 3 & 4 parameters are purely system properties dependent.

Hope this helps, please do let us know if any doubt you have.

 

P.S.- As Harvey said, there is no rule which states that DNCR area is ~ 10% of column area. DNCR area and Active area are dependent on vapor-liquid traffic inside the column such as vapor and liquid mass flow, density, viscosity and liquid surface tension. At high reduced pressure systems (results in high vapor density and very low surface tension) can have equal active and DNCR area in a column.

Edited by Padmakar, 18 September 2013 - 02:30 AM.

[abhi_agrawa]

Chane,

 

Assuming that your vapor to liquid ratio is correct, it may be worthwhile to consider 4-pass tray as the liquid load seems to be excessive.

 

Is it possible for you to post you KG-Tower file or your loads for us to verify?

 

abhishek

[a.aryan]

^{Hello Guys,}

 

^{The rule of thumb is that the SIDE downcomer WIDTH should be a MINIMUM of 10% of the column diameter. This gives a weir length of 60% of the diameter and a downcomer area of 5.2%. The reason for this rule is to avoid stagnant liquid zones at the sides of the tray.}

 

^{With respect to downcomer design:}

 

^{The purpose of a downcomer is to disengage entrained vapour from the aerated liquid.  In sizing the downcomer the two main criteria that need to be considered are:}

 

            ^{Downcomer “Choke” Flood}

            ^{Downcomer Back-Up Flood}

 

^{The downcomer choke flood should not be based solely on an entrance liquid velocity since it is not superficial liquid velocity per se that is critical but rather the liquid residence time in the downcomer.}

 

^{An entrance velocity of 0.13 m/s is a good starting point - for a standard non foaming system operating at less than 6 bar g with a tray spacing of 24”. We use a figure of 0.12 m/s (180 gpm/ft²) which translates to a residence time of 5 seconds.}

 

^{For optimising tray designs, the downcomer can be sloped up to where the bottom area is a minimum of 50% of the top area. The logic behind this is that the liquid at the bottom of the downcomer is less aerated than the liquid at the top thus requiring less residence time for disengagement. Sloping the downcomer as such would give a mean residence time of 4.4 seconds.}

 

^{The downcomer size (width) is de-rated pro-rata to the tray spacing.}

 

^{The downcomer size (width) is also de-rated pro-rata to the system or foaming factor.}

 

^{Consideration also needs to be given to the liquid through over the weir. This is quite often neglected. Ideally we do not want the liquid to strike the column wall (shell) until the liquid falls half way down the tray spacing. This will avoid premature choke flood. A method for calculating the liquid throw is given by W L Bolles (Optimum Bubble-Cap Tray Design, Part I – Tray Dynamics, Petroleum Processing, 1956, p64-80).}

 

^{The downcomer back-up (flood) is a function of the following:}

 

            ^{Dry pressure drop}

            ^{Wet pressure drop} 

            ^{Head loss due to the liquid flowing under the downcomer}

 

^{Calculated as clear liquid, the back-up should not exceed 50% of the tray spacing for a system operating at 6 bar g or less. For high pressure service above 24 bar g the back-up should be less than 35% of the tray spacing. A longer residence time is required at high operating pressure due the increased difficulty of disengaging the vapour from the liquid as their respective densities are much closer together.}

 

^{The head loss under the downcomer can be reduced by increasing the downcomer clearance. However, care needs to be taken as the downcomer clearance influences the liquid flow pattern across the tray which in turn can impact on the tray efficiency.}

 

^{Other methods of reducing the downcomer back-up are to use radius edge (tip) downcomer panels, inlet pans and also low pressure drop valves.}