11 replies to this topic

hi, im designing a flash tank for the removal of unreacted butadiene from a SBR rubber in a Emulsion SBR process.

the total inlet to the flash tank is: 58029.69414kg/hr (of which 6098.267666kg/hr is the butadiene). assuming 100% removal the vapour leaving the top of the tank will be 6098.267666kg/hr butadiene and the liquid leaving the bottom will be 51931.426474kg/hr.

i have done the mass balance and am on to doing the energy balance and chemical engineering design. but i am stuck at this point, i have looked through C&R volume 6 but ther is no information on how to produce a energy balance and chemical engineering design for the peice of equipment.

Can anyone point me in the right direction?

thanks

[Art Montemayor]

Aziz:

I'll help you resolve this problem - or whatever it is that you are trying to resolve. Allow me to explain:

If you read your post carefully - as one of your readers might - you will find:

I believe you can write good, proper English and you probably are well-educated and prepared in your native language as well; your thought process reveals that you are a well-prepared student. However, my first problem is in understanding your writing. I know you know basic grammatical rules and that this is no teenage “Chat Room in Cyberspace”.

Please don’t treat us like immature, uneducated teenagers. The use of such obvious mistreatment of the English language is OK if you are rebelling against society; but this is not a protest Chat Room. This is a serious engineering Forum. Misusing the English language with terms like: “im”, “i”, and failing to capitalize the start of sentences just makes it that much more difficult to understand your technical writing and thoughts.

There is no sense in citing a number like “6098.267666 kg”. I believe the accuracy of your mass balance makes only 1 kg significance as appropriate and you are absent-mindedly only copying a computer print-out. Don't forget that the computer is simply stupid; it doesn't know about significant numbers.

You are supposed to do the thinking and interpreting.

You state "ther is no information on how to produce a energy balance and chemical engineering design for the peice of equipment". I don't know what a "chemical engineering design" means and I doubt if it is defined in any text book. I know what a process design and a mechanical design are. Do you mean one of these? I believe you mean a "process design". If so, please state it as such.

If you want a process design – determination of the diameter, height, and internal characteristics of the Flash Drum - then you should resort to the classical Souders-Brown relationship. This equation has been discussed and explained numerous times in this Forum. A search by you should give you the necessary information you require. If you do the search and can’t find the information, return to this thread and I will give you a short seminar on how to design a Flash Drum.

I hope this advice helps you out in successfully expressing yourself and in resolving your problem.

Hello Art,

Thank you for your reply and sorry for the miss-spellings and grammar errors in my original post.

What I meant in the post was, I was finding it very difficult to find information on the process design for a flash drum. Since my last post, I have read through numerous posts in this forum and found relevant information and equations for the design of a flash drum.

I have made another post today regarding my calculations, results and my general approach of the design process, I will repeat it in this post if you have not read it yet.

My new post stated:

Hi, I was just wondering if someone could have a look at the answers I got for the dimensions of the flash tank:

The process is removing 6099kg/hr of butadiene from a mixture of SBR rubber, water and styrene which has a total flow of 58030 kg/hr.

I used the the Souders-Brown equation to determine the maximum allowable vapor velocity:

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

I got a value of 2.11 m/s as this is the max velocity, I set a velocity of the vapour at 1.2 m/s.

I then calculated the diameter of the tank and got 0.84m. Next I calculated the height for a vertical pressure vessel with a length-to-diameter ratio of about 3 to 4 and got a height of 0.63m.

From these values I calculated the volume which was 0.62m^3. To my understanding, this is the volume occupied by the gas. Am I correct?

I specified the volume of the gas to be 60% of the total flash tank. Therefore the volume of the liquid is 40%.

I calculated the volume occupied by the liquid = 0.4132m^3

The total volume = 1.0332m^3

From this I calculated the new height of the tank which = 1.864m

The new dimensions for a flash tank are:

D (diameter) = 0.84 m
H (height) = 1.864 m
V (volume) = 1.0332 m3

I was just wondering if my thought of that the initial diameter, height of the tank and volume calculated from the Souders-Brown equation is the volume occupied by the gas, is so is my specification of the vol. occupied by the gas equal to 60% a acceptable figure in industry. Thanks

[Art Montemayor]

Aziz:

I am very happy to read a product of your mind expressed in clear and legible writing. This last post of yours is a reflection of your ability and your willingness to share your clear thoughts with others - especially those that can be of help or assistance to your needs.

I try very hard to stimulate young students and engineers to strive to better express themselves solely because at my age I know all too well the benefits that await those engineers who learn and practice early how to communicate well and accurately. I also abstain from working out problems for students because I firmly believe that to do so imparts great harm and false expectations on students. In your case, I have decided to share a Design Workbook on Vapor-Liquid Separators with you and any other student who might be reading this. Please do not receive this as a solution to a problem, but rather as a tool to be used in resolving similar problems. You also can assemble this type of tool for your own use in other areas of application and you would be wise to follow the example and start collecting and assembling your own engineering workbooks on all the subject matter that you can.

To answer your specific concerns, note that there is no attempt to calculate what you call the "gas volume". The Souders-Brown relationship gives you the Maximum Allowable Superficial Vapor velocity through the separator.

With the Maximum superficial velocity you can calculate the separator cross-sectional area since you know the vapor density.

With the separator cross-sectional area known, you can then calculate the separator diameter.

Note that you must set or fix the heights of the liquid level in the bottom of the separator and that of the vapor disengagement space above the feed entry and above any internal device. This you do with empirical experience or recommended values. Use conservative values and practical, aesthetical L/D ratios for the vertical vessel. Normally, a value from 2.5 to 3.5 is used by most for the L/D ratio. This depends on the application and the nature of the separation. I often set the liquid level height to reflect the surge time required for my low level detection instrument to react in time before a gas blow-by case is generated by a leaky or failed open drain valve.

I hope this helps you out and gives you a complete understanding of the subject matter involved and that you retain and improve on this knowldege and convert it into a skill and an experience.

Art Montemayor
 Art__s_Liq_Vapor_SeparatorRev1.xls   119KB   962 downloads

[jprocess]

Dear Art:
I believe in design of vertical separators L & D values should be calculated independent of each other and finally we check and correct these 2 values according to typical L/D values for vertical separators.We use L/D values in design of horizontal separators.Am I right?
Also I have 2 additional questions:
1.Could you please share your valuable comments regarding to the design of feed line(2-phase) to flash separator?(including its significance and necessity of using devices for hammer control).
2.What are typical values for residence times between "control points" for example between high and high high level?

[Art Montemayor]

jprocess:

Yes, you are right. That’s exactly what the Souders & Brown relationship gives us: the ability to calculate the Maximum Allowable Superficial Velocity. Knowing this value, the remaining algorithm is a piece of cake. The length of the required vessel diameter is determined just as I stated: use empirical and experienced heights for the disengagement spaces required to separate the vapor phase from the liquid phase – without entrainment.

When dealing with flashing fluids, I always design my control valve’s location as close as is physically possible to the Flash Drum entrance nozzle. In fact, I nearly always flange up directly to the entry nozzle (which I try to have at least one size larger than the expansion valve. I never take the expansion flash in the line. Since this is a Student Forum and the thread is at a student’s level, I believe that discussing slug flow control and arcane subjects such as water or steam hammer is a bit beyond a student’s background of understanding and, as such, will only muddy the water for the students who can’t join in or possibly understand the complex issues yet. The subject of 2-phase flow is already a bit much for a ChE undergrad. I believe this is a subject for the Professional Forum – where it can be discussed with more freedom and less restrictions.

Typical response time for a control valve or similar final control element action depends on the type of transmitter, signal, control loop, and final control element. For a pneumatic-actuated block valve, I’ve used values from 10 seconds to 30 seconds. Every application should be studied carefully and accurately in order to ensure a safe closure. I don’t believe in generalizing this function because there is a very large variance in liquid capacitance between different applications. This is a typical application of “good engineering judgment” and the reason professional, experienced engineers should be consulted/employed when addressing this type of problem.

[Skyline]

Dear all,

I want to ask something about separator sizing. Is Souders - Brown equation applicable to all type of separator, just like a centrifugal separator or cyclone separator?

I have a data in a geothermal plant. According to the design document, the separator can handle 400 tonne/hr 2 phase fluid flow. But when i calculate it using Souders - Brown equation, i found it can handle only about 120 tonne/hr, a very significant different number isn't it?

I hope somebody can help me to explain it (May be the differences of calculation procedure for some types of separator). Thank you very much before.

Best regards,

Erwan

[shan]

Souders-Brown equation Vmax=K*(L - g)/g is essentially applicable to gravity separator. If you want to extend the equation to a separator using centrifugal force, how do you define Vmax for a centrifugal separator or cyclone separator? Center velocity or edge velocity?

azizi1:

hi
my friend , send sketch for the flash drum with caculation you have been done,and let every body trying

thanks

[jimbio]

Dear all,

I've a real silly question:

I.e., at the inlet of the flash drum the stream is a mixture of liquid (70%) and gas (30%),
but which is the correct flow rate for drum sizing?
- gas 30 % of flow
- liquid 70 % of flow
- or both 100% of flow

I think that the correct input on Art's flowsheet should be only gas flow rate.

Am I wrong? Could anybody confirm?

Thanks!

how to calculate volume tank, separator, stirrer ....?

[siva_hps]

Dear Art,

I like to Design a separator vessel for Steam water seperation, Pressure of Inlet (Water +steam Mixture) is above subcritcal (during initial) and supercritical. Will the same procedure of two phase vapour liquid seperation can be used?. If not Please explain the design methodology. (this is for one of supercritical Boiler) The Centrifugal force used in the vessel (b'cos of cyclonic effect) to separate.

Kind regards
K.Sivakumar