39 replies to this topic

[merac]

Hi
I'm a spanish student who is doing the Final degree project and I have a vacuum distillation column (0,2 bar) in my process and I have some doubts related to its design and control:
-CONTROL: 1. what are the options to control pressure?
-DESIGN: 2. what type of reboiler should I use?
3. what are the main important design points in order to assure a good operation of the column?

[Art Montemayor]

Merac:

1. Depending on what you are doing (you haven’t furnished any basic data or scope of work) the normal or conventional way to control the column pressure is by establishing the vapor pressure at the top of the column. This is done with the vacuum level.
2. You can use almost any type you desire. I’ve used both thermosyphon (AEM) and kettle (BKU) with success.
3. The main important point s are many. It would take a book to describe them. Please be specific in stating precisely just what it is that is troubling you or makes you feel uncomfortable or insecure in design the column. That way, we can get directly to the points that need to be addressed for your benefit and understanding. For example, are you referring to process design points or mechanical design points – or to both? There are books written on this subject and literally thousands of journal and magazine articles in the literature.

I have aided and helped many students in their final degree project in past years – 3 of them have been Spanish ChEs who put out a great report in the end. My experience with them has been that lack of hands-on and lack of experience in the practical operation and control of Unit Operations has given them an erroneous idea of how the actual design and operation really takes place. This hasn’t been their fault; it happens all too often to all ChE students because, in my opinion, of faulty teaching and technical training in the basics of major process equipment and the mechanical skills. But once this hurdle is overcome, the average ChE immediately enters a level of self-confidence and total awareness of what is occurring in major process equipment and how to understand and undertake such difficult job assignments as startups and shutdowns. These are such critical and essential points of knowledge to all ChEs and yet they are not normally taught in the major universities – mostly because the teaching staff has no hands-on experience – or interest.

Let us know what your specific needs are and in what specific areas we can be of possible help.

Saludos.

[merac]

I'm a spanish student of 5ºCourse of Chemical Engineering who is doing the final degree project .
The distillation column operate at 0,2 bar and its objective is to remove the methanol and water that a methyloleate stream has.
This column has a partial condenser and my problems are the following:
1. In order to do the vacuum the column needs, I have put a steam ejector on the vapor stream that leaves the reflux accumulator, but:
-what can I do to avoid the ejector pumping out not only the uncondensables but also the liquid?
-how can I control de vacuum I need? with the high pressure steam?
- where do the stream that exits the ejector (methanol and water) goes?
- In an other distillation column that operates also at 0,2 bar, I have a total condenser, so how can I make this vacuum if the stream is liquid?

2. In order to do the mechanical design of the column, I have to estimate shell thickness taking into consideration the pressure, but is it neccesary to put some reinforcements in the column to prevent accidents related with vaccum?
thanks for your help

[Art Montemayor]

Merac:

Please download and study the attached Vacuum workbook which I have prepared to train young engineers in the concepts of vacuum operation and related equipment.

You have located your selected steam jet ejector in one of the proper places where it could go. You can avoid the ejector pumping out not only the noncondensables but also the liquid by simply locating it on top of the Reflux Drum and preferably with a baffle welded inside the Reflux Drum, directly under the nozzle where the noncondensables exit. I normally avoid this location because you are forcing the Reflux Drum to function as a vapor-liquid separator as well as a liquid reservoir. I prefer to locate my vacuum device directly on the total condenser itself. Please refer to the 2nd attached workbook related to this thread to see the mechanical orientation and design criteria. Note the common sense employed in this engineering design. This way you don’t have to design the Reflux Drum as a separator – only as a reservoir. This is very typical of most – if not practically all – engineering involved in Unit Operations Equipment design and operation. There is no complex or nuclear physics involved here – just common horse sense. You will encounter this type of common sense decisions often should you continue on to a successful engineering career in the future.

You control the degree or quantity of vacuum level that you require for your process by simply oversizing your vacuum creating device (the steam jet ejector) and bleeding in atmospheric air a little distance upstream of your device. You carry this out by simply applying a control valve and a vacuum controller.

The vapor stream that is extracted from the system (what you call the methanol & water) as well as the non-condensables is emitted to either a vent condenser (for possible methanol recovery) or to the atmosphere as a potential pollution problem. The latter is one good reason why steam jet ejectors are not very popular nowadays. If you can’t tolerate the emissions, then select another type of vacuum device.

When you have a total condenser on a column it is much simpler to pull a vacuum. You can use either of the two methods I have described above. In this case the Reflux Drum still operates as a vapor-liquid separator – but only to a small extent.

It may not be necessary to reinforce the shell of the column in order to rate it for full vacuum design. Do not forget that you must consider the case where the column could go into full vacuum – either accidentally or by instrumentation failure. Common sense dictates that you design the column (& all attached vessels) for full (absolute) vacuum and rate them as such.
 Producing_and_Maintaining_a_Vacuum.zip   56.49KB   1784 downloads

Attached Files

•  Vacuum_Distillation_Column_Design_and_Control.xls   905KB   2523 downloads

[merac]

Thanks for your help, it has been an excellent explanation!
I am using the excel work you send me, about "producing_and_Maintaining_a_Vacuum.zip" to design the ejector but as I have to quote every book or article or webpage I use in my project, I need to know the reference of the work you send me, is it a book? Can you give me the reference in order to put in on my project?
Thanks

[merac]

I have been thinking on the second configuration, the one you prefer, and I don't understand the following aspects:
1.What is the equalization line for?
2. Why is the condensation stream of the condenser inmersed in the reflux drum? I suppose it is related to pressure aspects but I don't really understan.
Thanks for your help

[Art Montemayor]

Merac:

Prior Spanish Students (@ Alicante & Salamanca) that I’ve helped have referenced my personal notes and communications with respect to empirical data, equations, and personal experience and these have been accepted by the judges. You don’t identify the specific information that you plan to incorporate in your calculations and write-up, but I suspect it has to do with the empirical equations that I present for the determination of air seepage and leaks. You should identify this for what it is: empirical, field-tested and recommended relationships. Use my name and reference as the source. Anywhere else that you go to in order to obtain an answer to this type of problem will involve empirical factors and relationships, so you would have to reference them as well. There is no other practical way to skin this cat.

You state you don’t understand my second configuration. Please be specific in identifying it in order not to generate an erroneous response. I suspect you refer to my sketch of the partial overhead condenser. If so, then:

1. The equalization line is there to allow free and continuous drainage of the saturated condensate liquid from the condenser into the Reflux Drum;
2. A dip pipe is employed to route the produced saturated condensate into the Reflux Drum in order to ensure that there is a positive liquid seal between the condenser system and the Reflux Drum system.

Do not be embarrassed if you at first don’t understand a simple hydraulic operation such as free drainage of liquids from one vessel into another. Although this seems like an obvious “no-brainer”, it requires a thorough understanding of hydraulics and fluid mechanics. Many, many chemical engineering students and young graduates often make the mistake of under-estimating the design needs of a hydraulic system in order to ensure complete and efficient liquid drainage. This is one such example.

Look carefully at the sketch and note that I include details that you normally have not seen in chemical engineering text books. That is because the text books lack the practical and realistic industrial applications experience. Your system requires that the saturated condensate flow directly out of the condenser as soon as it is being formed in order to facilitate the heat transfer area to do its work continuously. That is why you will note that I employ the shell side of the condenser to condense the vapor and install multiple liquid drain nozzles in the bottom of the shell. Note that I purposely concentrate the majority of the drain nozzles towards the hot end of the exchanger. That is the zone where most of the condensate is going to be formed – and very quickly. As you travel further towards the cold end of the shell side, you are concentrating non-condensables and less liquid. Note that the flow driving force (pressure drop) is driving the vapors in that direction. We do not want any by-passing of vapor into the Reflux drum and therefore we establish a liquid seal between the condenser shell and the Reflux Drum by employing a dip pipe rather than just a simple nozzle at the top of the Reflux Drum. This allows only the liquid to drain into the Reflux Drum and keeps uncondensed vapors out. It also establishes a non-free-venting nozzle effect. In other words, the drain nozzle into the Reflux Drum does not have to be a “free-venting” nozzle. This is a natural hydraulic requirement that many students totally miss and remain ignorant of primarily because their professors fail to teach them the basic rules of fluid mechanics. In order to freely drain liquid from one vessel into another using gravity flow, you must take into account that the vapor displaced in the lower target vessel has to be either removed or transmitted back to the original, upper source vessel. Otherwise, the damned liquid won’t drain or it will “slug-drain” erratically! You cannot afford to make this mistake in a continuous process system such as a distillation column. What is commonly done is that an equalization line is installed, joining the vapor spaces of both the source and the target vessels. This allows the displaced vapor in the latter to be routed to the former and ensures steady, gravity drain flow.

As I have stated previously, what many professors fail to teach and to drill into Chemical Engineering students is the fact that the majority of their future designs and decisions will require common sense being applied in order to achieve engineering success. Engineering is the successful application of scientific principles on a practical and economic basis. Unfortunately, these basic truths upon which all engineers will be judged when they achieve professional status are totally left out of the academic text books and lectures in today’s universities. There either isn’t enough time, experience, or incentive for it to be done.

I hope you and other students understand now, by this example, why I always keep insisting in this Forum that engineering students practice the employment of common sense when applying engineering principles.

Also, before I forget to mention it, think carefully of how you must ensure the successful passage of any condensed liquid through the shell side of the condenser and through its internal baffles. By now, you obviously already know about heat exchanger baffles and the function they perform. Most baffles have a 25% “window”; that is, they are cut and have a circular segment that represents 25% of the exchanger’s diameter. If you are successful in visualizing the internal arrangement of heat exchangers, then you will realize that the lower (condensate collection zone) part of the shell side will be blocked by baffles – whether the baffles are oriented with the “cut” done vertically or horizontally. Automatically, you have a serious and grave hydraulic problem: you can’t transport the condensate without flooding some of the lower tubes (and effectively neutralizing their heat transfer capability)! You either have to notch the bottom of the baffles (a “Vee” notch is usually the custom) or install the system of drainage that I’ve sketched out for you in the workbook. As you can probably already see, there is a lot of common sense that has to be applied in doing a detailed engineering design.

Saludos.

[merac]

Thanks for your reply
I was designing the distillation column as a tray column (with 10 trays) but as it operates at vaccuum pressure (0,2 bar) I'm not sure if a tray column is the best decision because the trays produce a big pressure drop. Would it be better to design a pack column?
How is it usually decided if is better to design a tray or a pack column?
Thanks for your help

[merac]

I have decided to design the column as a packed distillation column because it produces a less pressure drop per theorical stage and that is very important when operating at vacuum.
However the design diameter that I've calculated is very small and I'm sure I'm doing something wrong but I don't know what so I'm going to explain how am I doing the design in case you can help me.
Objective of the column: remove water, methanol and triolein of the feed stream in order to have a liquid distillate with methyloleate 99,98 pure

Operating conditions of the column:
Feed Vap distillate L distllate Residue
Temperature (ºC) 60 162 162 321
Mass flow (kg/h) 3670 14,64 3471 184
Mass fraction
Methyl oleate 0,95 0,16 0,998 0,02
Triolein 0,05 2,64E-4 1,0E-03 0,98
Metahnol 3,00E-03 0,66 3,9E-04 0
Water 1,00E-03 0,18 3,0E-04 0

Type of packing: structured packing Flexipac HC 1,4Y(Koch) with an specific area of 350m2/m3 and 279 mm of HEPT.

To determine the diameter column:
-I'm using the maximum pressure drop criteria, and in my case, as I'm working with a vacuum system,
the maximum pressure drop per meter of packing is 8,6E-4 – 2,02E-3bar (Kister book about distillation design)
-with the data that is on the excel sheet I send you, I calculate the flow parameter.
-then, with the flow parameter and the maximum pressure drop I calculate the capacity parameter from a chart that is on the Kister book I mentioned before.
-with the capacity parameter I calculate the Cs value according to the equation:
Capacity parameter = CS* FP^0,5*kinematic viscosity^0,05, where CS is in ft/s, Fp in ft^-1 and kinematic viscosity in cst
-then, with Cs I calculate velocity us:
Cs=us*(gas density/(liquid density - gas density))^0,5
-and with velocity and volumetric flow, I calculate area and then diameter, and as a result my column has 0,065m of diameter!!!!! That is to small, and I don't know what I'm doing wrong
Please, can you help me?
I have also tried another criteria for designing the diameter column (the flooding velocity) but it has also given me a small diameter , and I don't know what to do

Attached Files

•  PACKED_COLUMN_DIAMETER_CALCULATIONS.xls   48KB   949 downloads

[Art Montemayor]

Sara:

I don't calculate a packed tower the way you say you learned in Kister's book. Frankly, I don't have much respect for Kister's book. But that's not important. The important points are that you can find helpful information and a definite solution to your problem. I strongly recommend you go to:

http://www.katmarsoftware.com/pcol.htm

and find out all you can about Harvey (Katmar) Wilson's program, "Packed Column Calculator". You can obtain a detailed illustrated manual and tutorial included in Word v6 format and the Registration cost is US$80-00 for a single user license. This is approximately 50% of what the Kister book probably cost you. Harvey will allow you to download a trial version for Free that you can try out. This program will resolve your problem in the shortest time and with the least effort. However, you won't learn too much about packed tower design and its details. To do that, you should get a copy of:

Random Packings and Packed Towers
Design and Applications
Ralph F. Strigle, Jr.
Gulf Publishing Co. (1987)
ISBN 0-87201-669-2

The generalized pressure drop correlation (Figure 1-15 or 1-15) can be found on p.17. This is the classical way and method that I learned to resolve packed tower problems and design packed towers.

To give you a good idea of what your diameter should be, you can read the short composition on how to estimate the diameter of a packed tower and this should be a good way to "zero" in on what the diameter should be.

I hope the above offers some help and that you succeed in defining your tower.

Buena Suerte.
 Diametro_de_Torre_Rellena.doc   68.5KB   512 downloads
 Packed_Column_Design.doc   263.5KB   1036 downloads

[merac]

Thanks a lot for the book you recomend me. I've already designed my column and I think it can be correct as the diameter is 3,5 m and its height 6m. and I have simulated in Aspen Plus and it calculates also a diameter of 3,5 aproximately

However, I still have to ask you more things because each time I begin to design a new process equipment of my final degree project, more doubts arise. Now, I'm designing the steam ejector.

I have to decided to put it on the noncondensables stream that leaves the partial condenser and following the rules that you give me in the Excel sheet call "Producing and mantaining vaccuum" I'm calculating the desgin capacity of the ejector by:
1. Calculate the air seepage.
Question1: To calculate the air seepage due to the system components I need to know what are the normal components that a packed distillation column has (type of static seal, type of dinamic seal, number of access port, viewing windows for a distillation column?)

2.Calculate the noncondensable or process gases
Question 2.. In the partial condenser, there is a vapor (14,64 kg/h) that contains 2,37 kg/hr of methyl oleate, 9,65kg/hr methanol and 2,62 kg/hr of water. What is the mass flow I should consider to calculate the ejector capacity? Only the methnaol and water flows or all the vapor stream?
Question 3: I thought that the steam ejector capacity was function of the vacuum pressure you need, but if you determine the capacity only with the air seepage and the noncondensables, how is the ejector capacity with the vacuum presure?
Question 4. Once I know the ejector capacity, are there any other important parameter to design the ejector? diameter? length?
Question 5. Related to the mechanical design of ejector, what aspects should I consider?

[merac]

I forgot to ask you another thing
How is the high pressure steam flow for the ejector calculate?

[Art Montemayor]

Sara:

1. Calculate the air seepage.
To calculate the air seepage due to the system components You need to know the quantity and types of nozzles, joints, flanges, and other pieces of equipment that are basic requirements for a packed distillation column. Sit down and think about what the column needs to operate efficiently and safely. For example, you need at least to consider the following: Feed inlet nozzle; bottom outlet; top vapor outlet; reboiler connections; safety relief valve; temperature probes; pressure probes; sample probes; manways for packing introduction and removal; level detection instruments; etc.. etc. Make a list after you’ve determined all the stream sizes and line sizes as well as the column diameter.

2. Calculate the noncondensable or process gases
Regarding the partial condenser’s, vapor flow, you should consider and establish just exactly what you plan to do with this vapor downstream of the column. Do you intend to condense it or to continue to transport it as a vapor stream to be used as such downstream? You haven’t told us your scope of work or explained what you are doing – other than just operating a simple vacuum still. I can’t comment or help you on this until you reveal everything you plan to do or are designing to do. Normally, people condense the overhead vapor stream and store it. Is that your case?

3. I don’t understand your question. I recommend you visit the Graham Website at: http://www.graham-mf.../ejlibrary.html where you can download and study all the information regarding the application of the steam jet vacuum system you propose. This should be of great help to you. Also note that Graham gives you a typical Specification Sheet to for the proposal of a steam jet. They are the ones that fix the steam requirements – not the user.

4. Refer to the Graham website that I mention above.

5. An engineer that is proposing to use an ejector does not get involved in its mechanical design. There is no logical or practical reason for this to happen. Unless you are studying or planning on entering the manufacturing of steam jet ejectors – which I seriously doubt – there can be no reason for you to get involved or to attempt to mechanically design one. I doubt that your professors expect you to mechanically design a steam jet ejector. They may rightfully expect and demand that you know and explain how it works and why – but it is totally impractical to get into the mechanical design of this type of specialty equipment. It would be the same thing if it were expected for you to mechanically design the pumps and compressors in your project. That is not done in the real world. Expert and recognized manufacturers do this in their sleep and they publish catalogs to allow you to select the correct size (capacity) of ejector that you desire. These catalogs also give you the expected steam consumptions of each of the different models. You should solicit this information from suppliers such as Graham – or from local Spanish manufacturers or suppliers. That is the way that industrial professional engineers do it and the way that I am sure your professors expect you to handle the problem.

[merac]

Thanks for your help
As you have asked me what is the objective of the distillation column, I will explain it.
I'm doing the final degree project about the design of a biodiesel plant from palm, soya, rape and sunflower oil, and methanol, and sodium hydroxide as catalyst; and what I have to design for my project is a distillation column to purify the methyloleate (biodiesel) that is produced in the reaction section, in order to reach the specifications of the european code EN-14214.
Because of that, the objective of the distillation column is to remove the methanol and water of the feed stream, recovering the methyl oleate in the liquid distillate with a purity that should be more than 96,5% (in mass).
This column has to operate at vacuum (0,2 bar) because the methyloleate degradates at temperatures > 150ºC and this is the reason why I'm asking a lot about how to operate with vacuum


The question 3 I asked you before about the steam ejector was the following:
-As I have calculated the capactity of the steam ejector (lb air/hr) with a figure from Ludwig that relates suction pressure with capacity for a single ejector system, I don't know what I have to do with the values calculated for the air leakage, and the noncondensables flow of the process.
What are the air leakage and noncondensables flow for?


I also have some questions related to the partial condenser and the reflux drum of the distillation colum:
Question1: Does the reflux drum always have an equalization line with the condenser?
Question 2: Does the reflux drum always operates with vacuum?
Question 3. Is it always necessary to calculate the condenser elevation with the equation that follows?elevation= 144*(receiver pressure-condenser pressure)/Condensate density
The reason why I'm asking you that, is because I'm a bit confused with the required elevation that the vacuum conderser must have.
If there is an equalization line between the vacuum condenser and the reflux drum, I suppose the reflux drum also operates with vacuum (at the same 0,2 bar of the condenser), so I cannot calculate the require condenser elvation with the above formula because receiver pressure and condenser pressure is the same. How can I calculate the require elevation in this case?

[Art Montemayor]

Sarita:

Thank you for the additional information and data. Now I have a better picture of what you are confronting and the scope. You still have not replied to my inquiry regarding the need to handle the overhead vapor product out of the partial condenser as a vapor. I have to presume that you must maintain that stream as a vapor entirely. I also don’t know if you are allowed to dilute this stream with the steam used as the motive force in the jet ejector. I have to assume that you have no problem with that. You of course realize that in order to transport the outlet vapor stream from the jet ejector you must furnish a driving force. In other words, you must either use a blower (or another jet ejector) or condense the vapor into a liquid. I assume you will either employ a blower or another jet ejector.

As I mentioned before, you seem to be – as most Chemical Engineering Students –confused and mixed up about what constitutes a vacuum system. I perceive that you are under the idea that you must evacuate process vapors in order to create a vacuum system. This is an erroneous idea and not the case at all. The prime factor that establishes the existence of a partial vacuum inside your distillation column is the vapor pressure existing in the top section of the column – which is the vapor pressure of the saturated liquid existing in equilibrium there with the exiting vapors. That is all you need to control in order to establish a partial vacuum in the column. It’s as simple as that. I will elaborate further after I address your additional questions:

Question 1: Does the reflux drum always have an equalization line with the condenser?
No, an equalization line can be substituted with a self-venting dip pipe. However, the diameter of the dip pipe will tend to be very big in order to facilitate self-venting. That is why it is much more practical to furnish a small diameter equalization external line to allow free drainage. Please study the hydraulics carefully as I have noted in the prior post. You are using only gravity to drain liquid from one vessel (the partial condenser) to another (the Reflux Drum). In order to carry out gravity drainage you MUST EQUATE the pressure in both vessels. This is elementary hydraulics and common sense. Otherwise, the liquid WILL NOT DRAIN! As I mentioned before, this is a point that many student s fail to understand or to take into consideration. Gravity drainage is taken for granted and not looked at as a hydraulic problem. While it is simple, it nevertheless requires an understanding and engineering design. You must vent (communicate or equate) the vapor space of the target vessel with the vapor space of the source vessel. Do not underestimate the importance of this hydraulic requirement or it will cause you much grief and your design will fail to work.

Question 2: Does the reflux drum always operates with vacuum?
Since a requirement for gravity drainage is that the two transfer vessels be joined through their vapor spaces, both vessels will have the same vapor space pressure – a partial vacuum in your specific case. This is very obvious and, as I have tried to explain, can be a source of much trouble to students who fail to see the need to design their application with strict hydraulic engineering principles. Both vessels have to be at the same vapor space pressure in order to achieve gravity drainage. It is common sense that dictates that the Reflux Drum cannot be at a pressure greater than that in the partial condenser – otherwise, the saturated liquid would not drain! How can a liquid drain from a low pressure to a higher pressure? (Stated another way: water does not flow uphill) And it is also common sense that establishes that the Reflux Drum cannot be at a lower vacuum than the partial condenser – how or where is the lower vacuum going to be achieved or what will produce it? The obvious answer is that a lower vacuum is not needed or desired. A simple equalization of both vapor space pressures will ensure that gravity alone will allow the liquid to flow from the higher vessel (partial condenser) to the lower one (Reflux Drum).

Question 3. Is it always necessary to calculate the condenser elevation with the equation that follows? Elevation = 144*(receiver pressure-condenser pressure)/Condensate density
I don’t know where you derive the equation for the “elevation”. The elevation difference between the liquid level in the partial condenser and the liquid level in the Reflux Drum is not calculated. As long as the condenser is above the Drum, you will have flow. You will also have a resistance to that flow in the form of the piping, the nozzles, and the entrance and exit losses. These you calculate (design) by using the customary fluid flow equations of resistance to liquid flow. The less the elevation difference, the larger the diameter of the respective drain pipe.

Now to elaborate more on what a partial vacuum system is and how it is created. As I’ve stated before, a partial vacuum in your case is nothing more than the vapor pressure of your liquid mixture’s vapor pressure at the temperature you have selected for the reflux in your tower. Of course, I’m stating the “ideal and perfect” condition of the partial vacuum. What really happens outside of laboratory conditions and in the real-life industrial application is that the components (the distillation tower, the partial condenser, the instruments, Reflux Drum, piping, flanges, valves, packing, gaskets, etc., etc. all are subject to leakage – however slight or small. As long as you insist on maintaining a vacuum in your system, the outside atmospheric air will try to invade your process - and it will succeed to the extent that you have imperfections and leaks in your components. This is inevitable and a reality of life. You cannot avoid it. If you could construct and maintain a perfect, 100% leak-proof system and your process did not generate gases (non-condensables) or have dissolved gases in the associated liquids, then you would not require a vacuum device! All you would have to do is establish your process temperature to correspond to the vapor pressure that you require (a partial vacuum) and keep it there. You would have a partial vacuum and it would stay as such because there would be no compounds (or gases) to raise the vapor pressure higher than what you control. Therefore, there is no need to “maintain” a vacuum with an external device (such as a jet ejector). However, as I stated earlier, that is not real-life and there will inevitably exist leaks and non-condensables will invade the system. That is why you have to calculate the air leaks and the non-condensables in anticipation of having to eject them forcibly with a device such as a vacuum pump or a jet ejector. You are not “ejecting” valuable process vapors (at least not intentionally); you are ejecting those “high-boiling” components that invade your system or are created within the system and tend to raise the vapor pressure. That is what is happening in a vacuum system.

I hope I have succeeded in explaining what I understand to be your difficulty in relating to what you have to design and how to go about it. Do not be discouraged. Far too many students struggle hard in this area of vacuum control because of bad training or lack of proper preparation in the basics involved – such as vapor pressure, partial pressure, and gravity drainage. It’s as simple as that. Please let me know if I've failed to address your concerns and to help you out.

Buen provecho.

[merac]

Hi
Thanks for your help
I am doing the mechanical design of the packed distillation column, and I have a doubt related to the manways.
I don't know the technical drawing of a usual manway:
-how are the attached to the vessel? with flanges?
-how are they openned in a quick way? what is the mechanism to open them?
thanks for your help

[Art Montemayor]

Sara:

The quickest and most accurate way to answer your questions is to direct you to:

http://www.tankmanways.com/

http://www.manways.co.uk/

http://www.manways.com/

http://www.lenapeforge.com/manways.htm

http://www.leeind.com/manways.html

Specifically, some answers are:

1. Manways are attached to steel vessels by welding (soldadura).
2. They are opened (quickly) in a variety of proprietary or common means - it all depends on your likes and dislikes as well as your experience with their sealing and operating characteristics. Some have ordinary hinges (bisagras) or davits; others have proprietary designs.

If you don't have to open them quickly (or if you demand a tight and leak-proof design) you can always employ standard flanges with a stud (macho) or bolted sealing mechanism with gaskets.

I hope this helps out.

[merac]

Hi
As I continue doing the project, more and more doubts arises, is a bit frustrating!
I'm calculating the minimum thickness of the distillation column and as it works at 0,1 bar I'm designing at full vacuum.
However the formula I'm using is:
thickness= (P*Ri)/(2S*E+0,4P) + C, where P is the design internal pressure in psi
the problem is that if P=0psi, the thickness will only be C, that is the minimun thickness because of corrosion problems, and that doesn't have too much sense
What does full vacuum means? what pressure value?
Thanks for your help in advance

[Art Montemayor]

Sara:

Go to:

http://www.cheresour...?showtopic=3297
Vacuum For Pressure Vessel Design

And read the full response given to Benoy. The important thing I would like to impress upon you is to not give in to frustration and surrender all efforts to dominate the subject of pressure vessel understanding and design. You are not alone in this endeavor. As I have implied before, many chemical engineering students suffer under the same deficiency or handicap – and not due to their inadequacies or short-comings. I mostly blame educators for not preparing Chemical engineers in a background of equipment and hardware with an opportunity to exercise hands-on training in the mechanical skills of welding, fitting, piping, and machine tools. I believe it is fairly obvious to you and I that there is a void in those skills in your background. But don’t get discouraged. These skills can be acquired and mastered – if you make it your business to do so.

Also read my response to figo on http://www.cheresour...amp;#entry11351

It is very important that you start to learn and apply the basic and necessary principles of always stating and identifying all your applicable references and sources of information when you state an equation or an engineering item of information. Without a detailed backup of source or reference, you are left without a reason to be taken as believable. Always cite your references or sources of information – such as your equation, together with all the detailed symbology identified and explained as to units.

Now go to:

http://files.asme.or...Series/9667.pdf

and download, print, and carefully read all of the excellent document you find related to the mechanical design and calculation of pressure vessels exposed to internal as well as EXTERNAL pressure. Note that mechanical engineers look at pressure differently than Chemical engineers do. They deal in gauge pressures and look at whether the net pressure imposed on a vessel is internal or external. They identify a lack of pressure inside a vessel as an EXTERNAL pressure – not as a vacuum.

This is an excellent summary over the design and fabrication of pressure vessels using the ASME code here in the USA. I don’t know what pressure vessel code you are using in your design, but I would think that you have all the necessary codes and norms required under Spanish law.

On page 113 you will find an excellent example of a mechanical calculation for a vessel under EXTERNAL pressure – a calculation very different from that for a vessel under internal pressure.

I think you will find this a great help in resolving your design problem and that you will soon discover that you are interpreting the equation you present in a totally wrong way. I believe you are mis-interpreting the symbols in the equation and applying it wrongly. The above document should clearly point you in the right direction – even if you are not using the ASME code. Although the code you apply may be different, it will certainly agree with everything calculated within the ASME code as well.

As we say in English: hang in there and don't give up. As you say in Spanish: Arriba, siempre arriba; y mana a la obra!.

Buen provecho.

[merac]

Thanks a lot for your help, all the documents are very useful but the third is the best

I'm going to read the chapter about the design for external pressure vessels, and I believe I will be able to continue with the information there.

I totally agree that even though I have try very hard during all the degree, and it has been successful, there are a lot of industrial aspects that I lack of, and it is because what I have studied in the degree is very theorical and far away from reality. I hope it will change some day

Thanks

[fallah]

Dear Mr. Art Montemayor

After studying your valuable explanations about equalization line and dip pipe in overhead condenser and reflux drum let me for presenting my viewpoints in this regards as follows:
If the downstream piping of the condenser which conducts the saturated liquid into reflux drum, is considered as gravity drainage system for gas entrainment, and we have the equalization line for equating the shell pressure of condenser and reflux drum,and also assume having entrained gas(you mentioned in your explanations as"displaced vapor"), then it appears that we can consider a simple nozzle at the top of the reflux drum rather than dip pipe.

I think establishing positive (or negative)liquid seal by dip pipe employed for separating two spaces with different pressures while those spaces have to be connected to eachother via liquid barrier.
Of course, i think that the gas volume entrained in dip pipe case is considerably lower than that of "simple nozzle" case , but with having the equalization line in both cases the entrained gas ,dispite of its volume,will be transmitted (came back) to condenser shell as source vessel.
Please correct my viewpoints if i am wrong.
Warm Regards
Fallah

[merac]

Hi
I'm doing the line sizing of all the lines in the project and I have a doubt related to this topic when the lines are at vacuum, as in my case, that some lines are at 0,1 bar.
I'm sizing the lines taking into consideration the maximun pressure drop criteria (instead of the maximum velocity criteria) because I work with vacuum and it is very important to minimize pressure drop in lines.
My question is: what is the maximum pressure drop recomended for different type of fluids and conditions (i.e liquid, vapours, pump discharge, etc. ), in order to select a correct diameter?

Thanks for your help in advanced

[Art Montemayor]

Fallah:

Welcome to our Forums. I notice that you are a new member and it is always a pleasure to have new engineering members in our Forums.

I believe we are having a problem with a small language difference. You state: “If the downstream piping of the condenser which conducts the saturated liquid into reflux drum, is considered as gravity drainage system for gas entrainment, and we have the equalization line for equating the shell pressure of condenser and reflux drum, and also assume having entrained gas (you mentioned in your explanations as "displaced vapor"), then it appears that we can consider a simple nozzle at the top of the reflux drum rather than dip pipe.” I believe you are avoiding the fact that when you require for liquids to flow purely by gravity from one vessel to another, there must exist an equalization of the vapor pressures in both vessels. Also, there is no “entrained gas” in this application. That’s exactly what is avoided with an equalization line. The same principle is applied in a self-venting nozzle. As I mentioned, you can also employ a direct drain nozzle from the condenser into the reflux drum (without a dip pipe) – but you must ensure that the nozzle is large enough to furnish self-venting (this is another way of equalizing the pressures in both vessels. Either way, you must supply an equalization of the vapor pressures in both vessels. I believe I stated that.

I don’t understand your statement: “I think establishing positive (or negative) liquid seal by dip pipe employed for separating two spaces with different pressures while those spaces have to be connected to each other via liquid barrier.”

You state “I think that the gas volume entrained in dip pipe case is considerably lower than that of "simple nozzle" case” and, as I mentioned, there is certainly no entrained gas in the dip pipe. I don’t understand how you assert that there is. There is a positive liquid seal established by the dip pipe, so there is no gas flow through the dip pipe. If there is no gas flow through the dip pipe, how can there be any entrained gas? There just isn’t.

I hope this explains what I have tried to describe.

[Art Montemayor]

Sara:

You are setting the proper, practical basis for sizing the pipes in your distillation column and related equipment. The maximum pressure drop should be that which is set by your configuration and process needs. You alone set these criteria with your design – both process and mechanical. Therefore, you will set what is the maximum pressure drop that you can tolerate through your piping.

Please refer to the attached workbook and study the outline sketch of your system – as I see it according to your description. Note the following:

1. You are to set the operating pressure at the top of the distillation column with your process calculations. This is your starting point with regards to the most difficult pressure drop to control – that of the overhead vapors all the way from the top of the column to the partial condenser and through shell side of the condenser. The most critical drop to control is the vapor flow from the column to the entrance to the condenser. Once the vapors reach the condenser, they start to condense and the pressure drop is less critical.
2. The saturated liquid reflux is flowing due to gravity, so this is rather simple and well controlled. Note that you must allow for a reasonable height elevation of the reflux drum from the reflux entry to the column. This, in turn, sets the height of the condenser and the height that the overheads vapor line has to travel before entering the condenser. You should sketch out the relative heights much as I have done in order to document the line lengths and the height requirements.

Pay careful attention to one very important point that you have not mentioned and which I know you will have to confront, calculate, and resolve: the height of the column required to yield a decent and controllable Net Positive Suction Head (NPSH) available for the bottoms pump. This is a very, very critical parameter and one that you will obviously have to explain to the jurado in the presentation of your project. From the 3.5 meter diameter you calculated for the column, you may be facing a tall structure (much like a steel frame building). Your reboiler will also probably have to be elevated in order to conform to the configuration of the column.

Keep your vacuum lines very short and direct in their path. Use a minimum of elbows or change of flow direction. Use long radius turns in your elbows. I recommend you employ the Darcy pressure drop equation for the overhead vapor line (as well as for all other lines) assuming isothermal flow and dividing the lengths involved into equal lengths such that the pressure drop calculated within each length is not more than 10% of the inlet pressure to that length. This technique (as explained in Crane Technical Paper #410) will allow you to employ the Darcy equation for compressible vapor flow and this should be explained as well to the jurado in your presentation.

 Distillation_Column.xls   93.5KB   843 downloads

[merac]

Thanks for your help.

I've just designed all the lines accordign to the maximum pressure drop criteria and taking into consideration usual velocities recommended.

Related to the NPSH calculations, I don't have to do that because my reboiler is a termosyphon and it works by natural circulation so I don't have any pump at the bottom of the column

I'm now doing the design specification sheets of all the equipment in my process, and I have a doubt related to the steam ejector. My question is: How can I know the pressure and temperature conditions of the discharge stream?

I've looked at a forum topic call: Ejectors and you say you have an Excel workbook related to ejectors, can you send me?


Thanks for your help