3 replies to this topic

[peaston]

2. A forced-circulation triple-effect evaporator using forward feed is to be used to concentrate a 10 wt % NaOH solution entering at 37.8°C to 50%. The steam used enters at 58.6 kPa gauge. The absolute pressure in the vapor space of the third effect is 6.76 kPa. The feed rate is 13608 kg/h. The heat-transfer coefficients are U1 = 6246, U2 = 3407, and U3 = 2271 W/m2 · K. All effects have the same area. Calculate the surface area and steam consumption.

Was given this question, which takes for ever to do... I am having trouble finding out the boiling point rises, BPR1, BPR2, BPR3, over each stage... We were given an example in class which was similar but the solution being used as a sugar solution, in which it was easy enough to get a bpr using an equation: BPR = 1.78x_1 + 6.22x_1^2, but i think for the case with this NaOH solution, we have to use Duhring charts...

Can anyone please give me some feedback on what they would do in this question

many thanks in advanced

andrew

[Art Montemayor]

Andrew:

You haven't submitted your work product on this problem. Does this mean you want us to produce it for you? Why don't you produce your work on a spreadsheet format so that you can show where you are stuck and we can see how you are proceeding to obtain the answer? That would be a fast, efficient, and effective way to transmit comments and suggestions to you on what you are doing well and what you are doing wrong.

[breizh]

Andrew ,

Process heat transfer by D.KERN should support your query .

Regarding boiling point , I've attached a link .
http://www.solvaycau...005_w_en_ww.pdf



Hope this helps

Breizh

[USR]

drew:
As am not currently at home i do not have my D Q Kern with me.I remember solving such questions in my college days.Your problem is with finding the boiling point elevation.Using the Duhring's plot is not difficult but there are somethings which you must know:
1.The system pressure in each effect,i hope you have probably calculated that.
2.The solute concentration of the solution from each of the effect.I hope you have done that too.
For your system pressure i.e. the pressure in the effect concerned obtain the boiling point of pure water,this could be obtained from steam tables.
Next enter the DUHRING'S plot from the bottom(the boiling point of pure water which you obtained from steam table) then trace up to the concentration percent concerned.
Now go left and read the solution boiling point on the vertical axis,the difference between the two temperatures is the boiling point elevation.