7 replies to this topic

[KeJoSa2010]

Hello,


I have been asked to size a horizontal 3-phase vessel with a boot based on a paper entitled "Successfully Specify Three-Phase Separators" by Bill Svrcek and Wayne Monnery. Anyway, the procedure appears straight forward; however, there is one part that I am struggling with. They say "For horizontal separators with a given diameter, the heights of the light and heavy liquids are assumed......". As far as I can see they do not recommend a basis for these assumptions and, I have no idea where to begin (I have no previous experience sizing a 3-phase separator). What must I consider when assuming light and heavy liquid levels? All suggestions and comments are appreciated.

Thank you,

Edited by KeJoSa2010, 02 August 2012 - 03:22 PM.

[Technical Bard]

Pick a residence time for each phase and select a normal interface level that gives you about what you think you want.

[Art Montemayor]

KeJoSa1050:

TechnicalBard is right on with his recommendations. This is the normal way such separators are designed and sized.

You have to be patient and understanding with Svrcek and Monnery, realizing that they are basically in academics and when they offer solutions to engineers, they are going to assume that you are going to take care of "all the rest of the details" - like having to assume a good logical, residence time for your fluids. The residence time best suited for your application is related to the type of crude and composition, as well as temperature and agitation. A residence time of 15 - 30 minutes is what I am accustomed to seeing, but of course the size (and cost) of the separator increases with every extra minute you add to the residence.

[KeJoSa2010]

Thank you Art Montemayor, and Technical Bard, I really appreciate the insight. I have a follow up question with respect to the same article and topic. There is one section in which they give instructions for designing the separator to include a heavy liquid boot. In there, they tell the reader to set the light liquid level in the vessel as well as the boot (if you have the article, see step 7 of the horizontal design procedure: heavy liquid boot on page 35). The diagram that accompanies this section (see figure 2, "interface control with boot" on page 31) appears to show a minimum height of light liquid in the boot of 6 inches. At this point in the procedure, we have not yet determined the diameter of the boot. Without the dimensions of the boot, how do I "set the light liquid heights in the vessel and the boot" based on the residence times? I am sure there is an easy explanation and that I am simply missing something; however, as before I appreciate any insight you can give me.

Thank you

[Art Montemayor]

KeJoSa1050:

I am frankly confused by your description of the “boot” and it’s importance. Please generate a detailed sketch of what is being discussed with clear and detailed notation on the parts that are giving you problems. That is the best and most accurate way to convey not only the problem, but also the solution.

A boot is not intended to make a simple phase separator more efficient or more complex. It normally serves as nothing more than an incremental addition to reservoir capacity inside the vessel. But the amount of extra reservoir capacity that it can lend is minimal. Ordinarily, there is a practical reason for going to the extra expense of installing a boot. In this case, I can’t figure it out. Please submit the requested sketch so we can have an accurate conception of what is being discussed.

[KeJoSa2010]

I have attached a representation of what I am talking about with my explanation.

Thanks again

Attached Files

•  Separator with a boot.pdf   87.83KB   207 downloads

Edited by KeJoSa2010, 31 August 2012 - 05:52 PM.

[Fr3dd]

Taking a look at the sketch, I can give you my opinion. Normally, when sizing a horizontal vessel, the volume required will be given by the flowrate to be handled and the residence time considered by the designer (which, at the same time, depends on the application of the vessel). There are also some guidelines about minimum distances that frequently are recommended or even sometimes specifically required by international standards.

For example, you can see the minimum distance recommended from the vessel top to the demister pad in the vapor outlet, or the distance from the bottom of the demister to the HLL. These figures are normally taken because they have been used for similar designs in the past and they have worked.

Specifying 1ft minimum from the vessel bottom to the low liquid level is OK (the outlet nozzle being at the same height of Low Liquid Level is not OK). In any case, this values are meant to be taken as basis for your calculations and, with a residence time properly chosen, will give you the diameter.

Hope this helps. Good luck.

[Art Montemayor]

KeJoSa1050:

Thank you for submitting the sketch. It says all that I was looking for in order to understand what you meant. As expected, this is yet another type of 3-phase separator. This one has no internal weir and depends solely on the capacitance of the boot furnished.

Fr3dd has presented a good and clear explanation of how the principles of separation are applied to this type of separator. I agree with all he has written – except the part about international standards. I have never found that there are any standards for separators – and that is why I couldn’t conceive what type of separator you were describing without a sketch to follow. Residence time and the content of each of the 3 phases is what determines the sizing and selection of the separator – as well as the dimensioning of the internal parts. In your separator, it is clear that the content of the heavy oil is relatively minimal and that is why you can get by without a weir and simply use the external boot as your heavy oil reservoir and level control point. Since the setting of the residence time for each of the phases is critical for the design to work, it is necessary to know – or to fix – the dimensions of the vessel and the boot.

At first glance, you might get worried that it is difficult to calculate the various volumes contained at differing heights inside a horizontal vessel with 2:1 ellipsoidal or F&D heads. It isn’t. All you need is a little free spreadsheet program, much like the one (Vessel Volume) that I have uploaded on our Forums a lot of times. With this spreadsheet, you can accurately calculate the volumes and residence times inside the separator. Look through our Forums with the SEARCH machine and you can design any horizontal vessel liquid level – using ellipsoidal, F&D, Hemi, or flat heads.