6 replies to this topic

[Gabriel Hunt]

Hi, I am a student currently designing a multi stage evaporator to concentrate caustic soda from 32 weight % to 50%. I have chosen a nickel fabricated calandria to resist corrosion, with backward feed double effect evaporators. I also plan to use a falling film tubular evaporator. These choices are based on information obtained from this article:
http://www.scribd.co...fect-Evaporator

Upon starting my calculations, I reached a small problem. Previously in exercises, I was given the values of the overall heat transfer coefficients, and calculations were carried out from there to calculate the area for the evaporators. However, in this case, where the U value is not given, how do I continue my design?

I did try searching for calculations for the U value, but the given equation:
http://www.engineeri...ient-d_434.html
cannot be solved without the area and wall thickness, values which I probably will only get with my mechanical design.

Searching further, I found a graph giving values for heat transfer coefficients. Can I use this?:
http://www.gea-phe.c...poration_en.pdf

Some other data about my evaporator:
caustic feed concentration: 0.32 weight %
caustic outlet concentration: 0.50 weight %

feed (caustic+water) flowrate: 170.87 kg/hr

steam inlet pressure: 1 bar
steam into second effect: 0.1bar

Thank you for your time and consideration, I will gladly provide any more information required to help me continue.

[breizh]

Let you get a copy of Kern :process heat transfer . It will support your query.
Breizh

[Gabriel Hunt]

Thank you for your swift reply. I will study it.

[kkala]

Another good reference is McCabe, Smith, Harriot, Unit Operations of Chemical Engineering, Chapter "Evaporation" (Chapter 16 in 5th edition of 1993), reporting two cases of concentrating NaOH solution as solved examples (single or triple effect evaporators).
It is assumed that you estimate a preliminary size of the evaporator, probably for cost estimate in a student project (precise data would require quotations from vendors). Feed 170.87 kg/h (small flow rate, check it) of solution 32% NaOH w/w, outlet 109.4 kg/h of solution 50% NaOH. Boiling point elevation (important for soda solutions) is written in mentioned reference book, as well as in other books.
Compare diagram of fig 2 in your reference to values written in the 2nd example of McCabe-Smith-Harriot. Probably difference is not significant, but both refer to evaporators of much higher capacity (check your inlet rate, it seems small for industry). I do not know what heat transfer coefficients can be used for such a small scale. At any case you have to assume overall heat transfer coefficients U1 and U2 (including fouling factors) from relevant data of literature. Never mind if they cannot be precise.
A way to solve the case: you have to assume boiling temperatures in first and second effect (*) and then estimate their heat transfer surfaces. If these are not practically equal (effects usually have equal heat transfer areas) you have to correct assumed temperatures and repeat the procedure (trial and error, more explanations in McCabe, Smith, Harriot's book).
Type of evaporator can be also based on information from this book.

Note (*): Reference book (5th edition) reports following temperatures of 50% NaOH solution
single effect 92 oC (197 oF)
triple effect 119 oC (246 oF)
relevant viscosities can be found in Appendix 9 of the book (viscosities of liquids).

Edited by kkala, 01 April 2012 - 12:49 AM.

[breizh]

I've added a link to support :
http://www.nzifst.or...evaporation.htm

Breizh

[INSTRUTECH]

REFER EVAPORATOR HANDBOOK

[jrtailor09]

Gabriel Hunt

When we start designing first question will come in my mind that Multipule effect evaporator required or single stage is enough for that ?

I feel it is not easy to decide. How much concentration of Caustic you are getting after first effect? There are lot of factor to be considered for deciding number of effect.

Find attached sheet from D. Q kern for the overall U in Red box.

Steam pressure unit shall be in guage right ?

Regards,

Jatin

Attached Files

•  U .pdf   154.4KB   123 downloads

Edited by jrtailor09, 24 May 2012 - 07:11 AM.