5 replies to this topic

[Propacket]

Dear All,

Please refer to the attachment.

I am sizing a TEG flash separator and my problem is what is exact method of choosing an optimum L/D ratio?
A well known paper by Svreck has specified various L/D values based on pressure as follows:

0<P<250 PSIG, L/D=1.5-3
250<P<500 PSIG,L/D=3-4
P>500 PSIG, L/D=4-6

Operating pressure of TEG flash drum is 65 psig. Therefore, i should use L/D ratio of 1.5-3.
What i know about choosing L/D ratio is that satisfying all the operating constraints (gas velocity, residence time etc),try different L/D ratios and choose the one that has the least weight of material. That is what i have done in the attached spreadsheet. These are software calculated values.
As you can see from the spreadsheet,L/D ratio of 6 has the least weight of material. Should i choose L/D value of 6?
Can i use higher L/D ratios (greater than 6)if all the operating constraints are fulfilled? What are limitations of choosing higher L/D ratios even if they give lower weight of material?

Attached Files

•  Separator.xls   20.5KB   522 downloads

Edited by P.Engr, 27 December 2010 - 06:04 AM.

[ankur2061]

P. Engr,

Shell DEP 20.04.10.10-Gen. (Glycol-Type Gas Dehydration Systems) recommends the following:

The vessel should normally be horizontal and sized for a liquid hold-up of 30 minutes in the inlet compartment. Since liquid capacity is limiting and gas rates are low, it should usually be designed to operate between about 60 % to 80 % full.

If I need to consider a horizontal design I would recommend a minimum L/D ratio of 3 for optimum disengagement of vapor and separation of liquid (TEG and heavier hydrocarbons (condensate)).

With vertical design, an L/D of 2 should suffice.

Hope this helps.

Regards,
Ankur.

[breizh]

HI ,

Let you try this resource :

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

Hope this helps

Breizh

[Art Montemayor]

P.Engr:

Frankly, I am very confused and disturbed by the apparent lack of basic knowledge application to this type of vapor-liquid separation unit operation. This subject – particularly the specific application of the Brown-Souders Relationship – has been expound, discussed, interpreted, dissected, and specifically detailed as to its application numerous times in our ChE Resources Forums.

One outstanding contributor to this subject in the past has been Milton Beychok, who has gone into great detail regarding the design of vapor-liquid separation devices. Just a few sources of his inputs are:

• http://en.citizendium.org/wiki/Vapor-liquid_separator
• http://www.chemicalforums.com/index.php?topic=8913.0
• http://www.chemicalforums.com/index.php?topic=9252.0
• http://www.cheresources.com/invision/topic/41-sizing-a-flash-tank-or-vapor-liquid-separator/
• http://www.eng-tips.com/faqs.cfm?fid=1153

In any vapor-liquid separator vessel, the resultant clean vapor with the specified liquid droplet size (as a maximum) should be the principal criterion employed in designing the vessel’s diameter. Once the vessel’s diameter is identified (as a minimum dimension), the subject of determining the vessel’s vertical height should next be resolved.

The height of a separator vessel should allow for the following vertical measurements:

• A height of liquid inventory in the sump of the vessel;
• A height of disengagement space between the liquid level surface and the entry of the 2-phase mixture;
• A height of disengagement space between the entry of the 2-phase mixture and the bottom of the internal mist separation device (either vanes or demister pad);
• A height between the internal mist separation device and the vessel’s outlet nozzle; this is normally used for maintenance and inspection of the internals and can accommodate a hand-hole or flanged access nozzle on the vessel’s wall. Sometimes, in the case of large vessels, the top nozzle can serve as maintenance and inspection entry.

Included in the above requirements should be allowance for such items as liquid slugs, liquid fluctuations, and the need for operational time required to react to liquid level or process variations, alarms, and instrumentation corrections. This allowance should be included in the vessel’s residence time. This is done because of the two phases – liquid and vapor – it is the liquid phase that poses a major threat if allowed to exceed maximum height values. The vapor phase is mitigated by a PSV, but the liquid phase has no “safety valve” except the proper operation of a level controller and if that fails, the downstream consequences of entrained liquid are usually a hazard. Whatever results as a "L/D" ratio is whatever results. I usually try to apply an L/D that fits in with the requirements for space, volume, and economical design. That usually - but NOT ALWAYS - winds up being something around 2:1.

Please submit your spreadsheet calculations for the presumed vessel you have calculated and how the L and D values were arrived at – as well as the vessel weight. This is to allow all of us on the Forum to see the specific calculation methodology and the results for ourselves. I am interested as to how you have arrived at that result. The best way to discuss this subject is to have the calculations in hand and not just assume that the correct and checked calculations have been done.

The main area of my concern in your post is that you supply a spreadsheet – but it is nothing more than a listing of some software outputs. It has no identification of the algorithms or logic employed in the calculations – and much less the calculations themselves. You don’t state what method or algorithm you are using to arrive at the recommended VERTICAL separator diameter. I have to presume you are using the Brown-Souders Relationship.

What doesn’t make common sense is that if you are calculating a VERTICAL separator, then the first answer should be the vessel’s MINIMUM diameter that makes for an acceptable MAXIMUM superficial velocity. That being the case, then what you have as a first answer is the DIAMETER OF THE VERTICAL VESSEL. This makes the diameter found as a FIXED, CONSTANT INPUT into calculating the height of the vessel. IN OTHER WORDS, YOU ARE NOT FREE TO VARY THE DIAMETER of the vapor disengagement space – unless you make it BIGGER than the minimum acceptable. – But if you are after the “optimum” vessel size, why would you want to make it bigger? Yet your calculation output shows that you are varying your diameter. This is not rationally and logically acceptable engineering. All you should be free to vary in this vessel is the HEIGHT OF THE VESSEL – not the diameter.

If you want to increase the liquid residence time for good conservative design, then you can make the sump (the bottom portion of the vessel) a bigger diameter. This does not change the superficial velocity of the vapor and gives you more liquid inventory. Or you can employ a “double-barrel” design that incorporates another vessel under the separator that receives the liquid separated and is itself subjected to a level control, leaving the main separator with only the jog of handling a vapor-mist mixed phase and subject to a maximum superficial velocity. If you are referring to Svreck and Monnery’s famous paper on vapor-liquid separators, then you are referring to the effects of the Brown-Souders Relationship and you should be totally aware that the basis of this relationship is the MAXIMUM SUPERFICIAL VELOCITY that can still give an opportunity for the liquid particles in the vapor phase to separate by gravity and settle down at the bottom of the vessel. That means you should not use a vessel diameter that yields a velocity greater than the Brown-Souders result. You can always use a lower velocity – BUT NOT ONE THAT IS HIGHER. Otherwise, you will entrain over liquid particles. That is what Messrs. Brown and Souders are trying to tell you with their famous equation.

Above everything else, DO NOT RELY ON A “RULE OF THUMB” BASIS TO JUSTIFY YOUR CALCULATIONS. This will simply “not fly” during a Hazop review. You need to have a calculation complete with references and sound engineering bases - such as the time requirements allowed in calculating the liquid residence time within the separator. For example, WHERE and HOW is this TEG separator operating? Is it on an offshore platform that operates automatically and normally un-manned? Or are operating personnel readily available? Does the vessel have redundant, back-up level controls and alarms installed on it? Or does it rely on only one level control device? All these factors (and others) enter into deciding on a sound judgment about how much residence time should be allowed in this vessel. This is compulsory and required common engineering sense that should be applied to this type of design.

I certainly hope this experience helps you resolve this query.

[Propacket]

Dear Mr. Art,

Thanks for your exhaustive response. Your response is a typical example of engineering knowledge that is merely earned by experience and this is what we can’t find in a textbook.

I had already taken a look at you’re earlier posts related to vapor-liquid separators & L/D ratio and have learned so much from them. Though I have developed the spreadsheet myself, I can’t upload the spreadsheet on forum because the spreadsheet is property of the company I work for. However, I can send you the spreadsheet through your email. Please tell me your email address. Plz avoid sharing with others. I hope you can understand this.

You are very right that we should not stick to rules of thumb to reach an optimum design. But you will appreciate that our world is full of people who seriously rely on rules of thumb they have learned from their experience. Let me give you an example. A rule of thumb for calculating duty of the amine reboiler is often employed that is ratio of lbs of steam to gallons of amine should equal unity for fine regeneration of the amine (to reduce the lean loading upto the required value). While working on a project, after process simulations and detailed review of the simulations, we found that the above ratio could be reduced to less than one to optimize reboiler duty. But our client completely rejected the design saying that they wanted a unity value for the ratio even though we had offered them process guarantee. Subsequently, we had to change the design completely. This is a typical example of the circumstances when we need to stick to the rules of thumb.

I agree that, by finding minimum diameter and various vertical measurements, L/D ratio is a result. But, as you will see in the spreadsheet, calculating the minimum diameter by Souder Brown’s equation and providing the required vertical measurements, L/D comes out 71.26. This strangely high value is due to very small diameter because of small amount of flashed vapors. Now we can reduce L/D upto the required value only by increasing diameter and adjusting vertical measurements based on the calculated diameter. Now in this case, what is the maximum value upto which we can reduce the L/D? Some references say that maximum L/D ratio employed is 6 for optimum design. But I don’t know why higher values can’t be used. I have programmed the spreadsheet for calculations based on various L/Ds. The spreadsheet calculates weight of the material for various L/Ds that helps judge the amount of material that would be used for fabrication. This is achieved by iterative calculations in visual basic. Currently, the spreadsheet is programmed for values upto 6. But I can increase the values if allowed. The spreadsheet calculations may seem unusually large and complex. If you need some explanation, please let me know.

Thanks

Edited by P.Engr, 27 December 2010 - 05:56 AM.

[Propacket]

Art,

please tell me your email address so that we continue or discussion.