Chemical Forums
Specialty Chemistry Forums => Chemical Engineering Forum => Topic started by: Starlord on July 02, 2016, 07:05:11 PM
-
Hi all
Please help
I have a question regarding calculating heat load and required surface area for a single effect evaporator.
Details:
Solution Feed rate 1.1kg s-1
Solution feed concentration 8%, concentrated to 30%
Feed solution at 60 degrees Celsius with Specific Heat Capacity of 4.2kJ kg-1 K-1
Boiling point elevation of 30 degrees Celsius
Heating steam is provided at 1.4bar and separator pressure is 0.1 bar
Condensate collected at rate of 1kg s-1 (assumed to be at condensing temp)
Overall heat coefficient 3000W m-2 K-1
Calculate:
Vapour flowrate:
w c₁ = (w - W) c₂
w = solution feed rate kg s-1 1.1
c₁ = feed concentration kg solute per kg solution 0.08
W = Vapour flow rate kg s-1
c₂ = product concentration kg solute per kg solution 0.30
1.1 x 0.08 = (1.1 - W) 0.30
W = 1.1 - (1.1 x 0.8)
0.3
W = 0.8066 kg s-1
Product flow rate:
w - W = Product flow rate
1.1 - 0.8066 = 0.2934 kg s-1
Product flow rate = 0.2934 kg s-1
These I am confident with.
-
Below I am unsure of.
Guessing I am wrong though but would require some explanation as to where I am going wrong please:
Calculate
Required Heat Surface Area:
Using Q = U A ΔT
Q = Heat Load
U = Overall heat transfer coefficient 3000W m-2 K-1
A = Heat Transfer Area
ΔT = temp difference between steam and solution 109.3 - 90 = 19.3 degrees C
A = Q
U ΔT
Q = m x hfg
where m = mass of steam condensed 1kg s-1
hfg = latent heat of vaporisation 2232kJ Kg-1
Q = 2232000W
so
A = 2232000
3000 x 19.3
A = 38.55m2
Something does not seem right here?
-
It goes on to request evaporator economy
Economy = Vapour product
steam supplied
=0.2934
1
= 0.2934
Again, appears suspect?
Other parts require to calculate heat required to raise the feed to boiling temp - I have attempted this but don't have the workings to hand
And available heat for evaporation and hence evaporation rate expected (I think this would be far easier if my previous sections were correct)
Please help asap.
I have atleast tried and am not asking for the actual answers but a clear demonstration and explanation to clarify where I am wrong
Thanks in advance
-
Why do you think your answers are wrong?
Look at the worked examples here: http://www.nzifst.org.nz/unitoperations/evaporation1.htm
Put your own numbers in and see if you still get the same answers.