15 replies to this topic

[ProcessEng_12]

Hello all!

I want to calculate the temperature and pressure of mixed gases which are mixed at different temperature and pressure.

The gases are HCL, Ethylene and air. Please help.

[shan]

T(mix) = (T1*M1*Cp1+T2*M2*Cp2+...+Tn*Mn*Cpn)/(M1*Cp1+M2*Cp2...+Mn*Cpn)

T: Temperature, C

M: Mass, Kg

Cp: Heat Capacity, KJ/(Kg*C)

[ProcessEng_12]

T(mix) = (T1*M1*Cp1+T2*M2*Cp2+...+Tn*Mn*Cpn)/(M1*Cp1+M2*Cp2...+Mn*Cpn)

T: Temperature, C

M: Mass, Kg

Cp: Heat Capacity, KJ/(Kg*C)

Is this formula for ideal gases?

The temperature of the mixed gas is not matching with the value obtained from above formula.

The gases are HCL, Ethylene and Air.

[breizh]

nishant ,

Better to show your calculation with input data , if help is needed .

 

Breizh

[heidar]

Dear Nishan

 

is it necessary to use thumb calculation for this purpose?

 

if there is not, i suggest you to use a strong process and chemical software such as Hysys, HTFS or something else.

 

BST RGDS

[shan]

The calculation discrepancy may be because of condensation or vaporization from fluid mixing or your fluid conditions are far away from the ideal gas conditions.

[ProcessEng_12]

nishant ,

Better to show your calculation with input data , if help is needed .

 

Breizh

HCL

Pressure:8.5 kg/cm2a

Temperature: 165 degree Celsius

Mass flow: 5200 kg/hr

 

Air

Pressure: 8.7 kg/cm2a

Temperature : 160 degree Celsius

Mass flow: 5400 kg/hr

 

Ethylene

Pressure: 10 kg/cm2a

Temperature: 120 degree Celsius

Mass flow: 2160 kg/hr

 

Pls help sir

[ProcessEng_12]

Dear Nishan

 

is it necessary to use thumb calculation for this purpose?

 

if there is not, i suggest you to use a strong process and chemical software such as Hysys, HTFS or something else.

 

BST RGDS

Yes sir But I think sometimes its better to learn from fundamentals

[ProcessEng_12]

The calculation discrepancy may be because of condensation or vaporization from fluid mixing or your fluid conditions are far away from the ideal gas conditions.

Thanks shan for your suggestion

But there is no condensation or vaporization.

[Pilesar]

Did you include the heat of reaction in your calculation?

[ProcessEng_12]

Did you include the heat of reaction in your calculation?

 

Sir, the reaction occurs in the presence of catalyst only. So can't consider the heat of reaction.

[MrShorty]

 

I've been out of the loop for a couple of weeks, so I realize this is old and the OP may not be monitoring the topic. Is there enough information to solve the problem? We have T and P for each input gas, but we re not given anything, P in particular, about the final gas mix. Is the final P intermediate between the three pressures, or is the final pressure below the lowest pressure, or are there compressors present so that the final pressure is higher?

 

It seems to me that, if we have enough information about the final mix (maybe P, density, and composition), we could compute T from an EOS.

 

PV=zRT where P and V are given and z is computed from an equation of state, then solve for T.

 

It seems to me that we cannot find the final temperature by any method unless we are given at least the final pressure.

[HomerMTY]

Try Gaseq, is a free program for chemical mixtures.

 

http://www.gaseq.co.uk

 

Best regards.

[ProcessEng_12]

 

 

I've been out of the loop for a couple of weeks, so I realize this is old and the OP may not be monitoring the topic. Is there enough information to solve the problem? We have T and P for each input gas, but we re not given anything, P in particular, about the final gas mix. Is the final P intermediate between the three pressures, or is the final pressure below the lowest pressure, or are there compressors present so that the final pressure is higher?

 

It seems to me that, if we have enough information about the final mix (maybe P, density, and composition), we could compute T from an EOS.

 

PV=zRT where P and V are given and z is computed from an equation of state, then solve for T.

 

It seems to me that we cannot find the final temperature by any method unless we are given at least the final pressure.

 

According to the indication given it is 7.2 kg/cm2a.

[MrShorty]

 

Your question takes me back to undergraduate P-Chem, which has been a long time ago. It has, therefore, been quite some time since I have done these kind of calculations rigorously, so forgive me if I forget something.

 

It seems to me that there are 2 steps to the mixing (I don't know if we can treat them separately or not). There's an expansion step where each gas expands from its initial pressure to the final pressure. Then there is a "mixing" step at constant pressure. For ideal gases, it seems that we can treat these steps independently. For real gases, the derivation will naturally be more complicated.

 

I might suggest this page (or any similar treatment from a P-Chem text or course) http://farside.ph.ut...res/node53.html  to refresh your memory on how to treat the expansion step. Recognizing that these equations are derived for an ideal gas, they should at least help recall how these are derived. You might also review the derivations around the Joule-Thompson effect, if that is a better representation of the expansion you are using.

 

Then, the equation Shan gave should give the effect of mixing the three gases at constant pressure, assuming there are no "interactions" between the molecules.

 

It still seems like a fairly idealized calculation, and I am assuming that we can treat the effects of expansion and mixing separately. See if that helps get you pointed in the right direction.

 

 

Edited by MrShorty, 11 August 2015 - 10:19 AM.

[ProcessEng_12]

 

 

Your question takes me back to undergraduate P-Chem, which has been a long time ago. It has, therefore, been quite some time since I have done these kind of calculations rigorously, so forgive me if I forget something.

 

It seems to me that there are 2 steps to the mixing (I don't know if we can treat them separately or not). There's an expansion step where each gas expands from its initial pressure to the final pressure. Then there is a "mixing" step at constant pressure. For ideal gases, it seems that we can treat these steps independently. For real gases, the derivation will naturally be more complicated.

 

I might suggest this page (or any similar treatment from a P-Chem text or course) http://farside.ph.ut...res/node53.html  to refresh your memory on how to treat the expansion step. Recognizing that these equations are derived for an ideal gas, they should at least help recall how these are derived. You might also review the derivations around the Joule-Thompson effect, if that is a better representation of the expansion you are using.

 

Then, the equation Shan gave should give the effect of mixing the three gases at constant pressure, assuming there are no "interactions" between the molecules.

 

It still seems like a fairly idealized calculation, and I am assuming that we can treat the effects of expansion and mixing separately. See if that helps get you pointed in the right direction.

 

Sorry for the late reply Sir. I read the complete concept which you told and understood it quite well. Thanks.

I used the formula which Shan gave. According to that the mixing temperature is 146 degree Celsius against 130 degree Celsius which is actual.