5 replies to this topic

[JasonT]

Hi all.

 

I am a chemical engineering student from Malaysia.  I was doing an isenthalphic flash question and the formula given to calculate Dh=-h1_dep+Dh_ig+e*h_dep(2v)+(1-e)*h_dep(2l)

 

where

 

1 is the inlet flow

2 is the outlet flow

e is the vapor fraction of the outlet stream

(1-e) is the liquid fraction of the outlet stream

 

h_dep(2v) is the departure enthalphy for the vapor of the outlet flow

h_dep(2l) is the departure enthalphy for the liquid in the outlet flow

 

I know that Dh_ig=h2_ig-h1_ig and I have calculated h1_ig.

 

My question is, how should I calculate h2_ig (two phases)?

 

I've tried using h_ig=e*h_ig(v)+(1-e)*h_ig(l), but when I use Goal Seek to determine the outlet temperature by keeping Dh=0, I got e more than 1.0, which is unacceptable.

 

Other details:

 

P1:40 bar

T1: 490K

 

P2:12 bar

T2 is the one I need to determine

 

Species present:butane, pentane, hexane, heptane

 

Thank you

Edited by JasonT, 21 December 2013 - 04:33 AM.

[PaoloPemi]

perhaps it would be useful some good textbook as "Properties of gas & liquids" or "Phase equilibria in chemical engineering" which probably you'll find in university's library,

 

for solving a flash operation with specified H you need a procedure to calculate H at inlet and outlet conditions,

simulators do use EOS (or equivalent methods) calculating H as

H = WfractLiquid*Hliquid+Wfractvapor*Hvapor

where Hliquid and Hvapor are calculated from ideal gas (or liquid) plus departure (via EOS)

note that things are not easy if you wish to include (as usual) phase equilibria,

with phase equilibria you need to solve the additional nc equations which define vapor and liquid individual fractions.

 

If you have coded your procedure in Excel (VBA) I would suggest to compare the values

with a industrial code, you may have access to a simulator (at university)

or you may download PRODE PROPERTIES which includes a very stable HP flash (multiphase)

and gives the option to calculate individual enthalpies directly in Excel so that you can compare with your values.

[JasonT]

Thank you, Mr Paolo.  I've used HYSYS v 7.3 to determine the outlet properties and that's why I was stuck with this question.  My computation shows otherwise.

[PaoloPemi]

if you wish to solve H + phase equilibria at some specified outlet pressure you have many variables to solve (fractions of each component + phase fraction + temperature),

if you are coding a procedure you can start with a double loop, in internal loop you calculate phase equilibria (fractions of each component + phase fraction) given a temperature, in external loop you estimate a new temperature based on enthalpy error and iterate.

"Phase equilibria in chemical engineering" has numerical examples as other resources which are easily available,

but you need to understand the details of the procedure,

as alternative you can use a simulator which does the work for you.

[JasonT]

Sir, here's another question.  When calculating the h_IG for both phases in the outlet stream, my friends used the mole fraction given for the inlet stream: nC4(0.25), nC5(0.20), nC6(0.25), nC7(0.30) and this got him the right answer confirmed by HYSYS.  

 

I solve the Rachford-Rice equation beginning the calculation for the outlet stream to determine e(vapor fraction), then i calculate the mole fractions of the four species in both phases so that I can calculate the h_IG using Gibbs.  But my answer is different, and therefore wrong.

 

Shouldn't we be using the mole fractions for the outlet stream instead of the inlet?

 

Or, is it that since I am looking for h_IG for the outlet stream, which has two phases (not just vapor but liquid as well), so there's actually no h_IG for the liquid part of the outlet?

Edited by JasonT, 22 December 2013 - 08:13 AM.

[PaoloPemi]

I am a bit surprised that you ask about mole or weight fractions (which are both acceptable depending from the way you calculate H or another property) as that means limited knowledge of the fundamentals,

since you are a chemical engineering student I suggest that you first learn the basis then try to code the procedure...

 

Anyway working in Excel and presuming you have coded the double loop procedure suggested in my previous post,

you can first validate the internal loop (Vapor-Liquid Equilibria) by comparing vapor or liquid fractions with some other code

using ideal model (based on vapor pressure) as this is very easy to verify,

(in Excel I use PRODE PROPERTIES which allows to enter both weight and mole fractions so that you can observe the difference)

 

once you have validated the phase equilibria section you have to validate the enthalpy calc's,

start by comparing your values (with different vapor and liquid amounts) with some industrial code

as PRODE PROPERTIES or another simulator, 

also in this case you can start with ideal model (no departure calc's) and verify that values

are about the same, you may adopt

 

Htot = WfractLiquid*Hliquid+Wfractvapor*Hvapor

 

where Hvapor = SUM(Wi*Hgi)

 

in a similar way you can calculate Hliquid

 

Finally you have to converge the external loop, you may use a simple procedure as for example bisection which is easy to code, or, as alternative, the Excel solver.

Edited by PaoloPemi, 23 December 2013 - 04:55 AM.