8 replies to this topic

[Luc1]

since nobody responded to my previous topic now i will ask something less difficult.

I have a mixture composed by CO2,H2,H20 at 1 atm and 25°C

This mixture is sent to a flash separator where gas and liquid phases are separated,now my simple question is this:

is possible that hydrogen can pass into the liquid phase before the mixture is sent into the flash?

thx

P.S. I'm spending the day searching for the Antoine coefficients for compounds like H2 and H20 but i found only the coefficients for CO2 and organic compounds, i have the perry's chemical handbook(7th editon) but I could not find them

Edited by Luc1, 08 September 2009 - 11:09 AM.

[MrShorty]

since nobody responded to my previous topic now i will ask something less difficult.

I have a mixture composed by CO2,H2,H20 at 1 atm and 25°C

This mixture is sent to a flash separator where gas and liquid phases are separated,now my simple question is this:

I'm having trouble visualizing exactly what you are doing, but I will throw in some comments.

is possible that hydrogen can pass into the liquid phase before the mixture is sent into the flash?

thx

Short answer: yes. Virtually all gases exhibit some solubility in liquid solvents. In the case of H2 in H2O, that solubility is probably small, but there will be some H2 in the liquid. Quantitatively, the simplest expression that describes the solubility of gases in liquids is Henry's law. I would suggest, to help understand the scenario you describe, that you brush up on Henry's law and locate Henry's constants for these gases in H2O. Recognize that CO2 is more difficult to treat rigorously because of the formation of carbonic acid, but you should still be able to see how to describe the solubility of CO2 in this way.

P.S. I'm spending the day searching for the Antoine coefficients for compounds like H2 and H20 but i found only the coefficients for CO2 and organic compounds, i have the perry's chemical handbook(7th editon) but I could not find them

I'm almost certain Perry's has vapor pressure data for H2, CO2, and H2O. If not Antoine constants, there should be vapor pressure data from which you can derive Antoine constants.

However, Antoine constants (or any equation for the vapor pressure) for H2 have no meaning at 25 C, being so far above the critical temperature of H2. Antoine constants for CO2 have questionable utility, as the Antoine equation is usually only considered adequate up to a vapor pressure of about 2 atm, which would occur well below the triple point for CO2. Unless you are being constrained to use the Antoine equation, I would suggest a different equation (Wagner equation seems to be a popular one) for the vapor pressure of CO2 at 25 C. You should have no trouble using the Antoine equation for the vapor pressure of H2O.

[Luc1]

This is the facility with which I am dealing :



Now im stuck at the 12 flow, the machinery after is the flash separator, the data i have on the 12 flow is this:

it was a liquid mixture composed by water hydrogen and CO2(11 flow), then it pass through a valve and reach the temperature of 25°C and 1 atm.

then it gives me this extra data:

12 flow: H2(%)=0.027 CO2(%)=0.37 H2O(%)=? Total-flow=?

13 flow: H2(%)=? C02(%)=? total flow=? T=25°C P=1 atm
then it says that in the 13 flow,which is composed only by gas, the CO2/H2=11.5

14 flow: H2O(%)=? CO2(%)=? total flow=? T=25°C P=1atm
i assumed that in the 14 flow(that is composed only by liquid phase compound)there is no H2 in the water mixture.

knowing this i must obtain the complete data of the 12,13 and 14 flows.

[MrShorty]

This is the facility with which I am dealing :



Now im stuck at the 12 flow, the machinery after is the flash separator, the data i have on the 12 flow is this:

it was a liquid mixture composed by water hydrogen and CO2(11 flow), then it pass through a valve and reach the temperature of 25°C and 1 atm.

then it gives me this extra data:

12 flow: H2(%)=0.027 CO2(%)=0.37 H2O(%)=? Total-flow=?

13 flow: H2(%)=? C02(%)=? total flow=? T=25°C P=1 atm
then it says that in the 13 flow,which is composed only by gas, the CO2/H2=11.5

14 flow: H2O(%)=? CO2(%)=? total flow=? T=25°C P=1atm
i assumed that in the 14 flow(that is composed only by liquid phase compound)there is no H2 in the water mixture.

knowing this i must obtain the complete data of the 12,13 and 14 flows.

Since this is posted in the student forum, I'm going to assume this is a homework type problem, and I don't want to short circuit the learning process by doing your homework for you.

Can you explain what it is about flow 12 that you are stuck on?

Perhaps something basic to get you talking about the problem. Can you explain to me what Henry's law is? Even if you can't calculate the numbers, can you explain how Henry's law applies to the flash separator in question? Can you use Henry's law to determine whether or not the assumption of no H2 in 14 is a good assumption or not?

Edited by MrShorty, 09 September 2009 - 11:37 AM.

[Luc1]

This is the facility with which I am dealing :



Now im stuck at the 12 flow, the machinery after is the flash separator, the data i have on the 12 flow is this:

it was a liquid mixture composed by water hydrogen and CO2(11 flow), then it pass through a valve and reach the temperature of 25°C and 1 atm.

then it gives me this extra data:

12 flow: H2(%)=0.027 CO2(%)=0.37 H2O(%)=? Total-flow=?

13 flow: H2(%)=? C02(%)=? total flow=? T=25°C P=1 atm
then it says that in the 13 flow,which is composed only by gas, the CO2/H2=11.5

14 flow: H2O(%)=? CO2(%)=? total flow=? T=25°C P=1atm
i assumed that in the 14 flow(that is composed only by liquid phase compound)there is no H2 in the water mixture.

knowing this i must obtain the complete data of the 12,13 and 14 flows.

Since this is posted in the student forum, I'm going to assume this is a homework type problem, and I don't want to short circuit the learning process by doing your homework for you.

Can you explain what it is about flow 12 that you are stuck on?

Perhaps something basic to get you talking about the problem. Can you explain to me what Henry's law is? Even if you can't calculate the numbers, can you explain how Henry's law applies to the flash separator in question? Can you use Henry's law to determine whether or not the assumption of no H2 in 14 is a good assumption or not?

There is something that i forgot to mention(my fault)in my post that was really important to solve the problem, i have the hernry constants for both H2 and CO2(given by text), then i already know the basic equations for the flash separator and are this one:

henry law for the flash:

P_tot*Y_H2=H(H2)*X_H2 same for CO2

global balances:

z*F=V*Y_H2+L*X_H2 this formula can be also written for CO2

Y_H2+Y_CO2=1

F=V+L

Where F is the total flow of stream 12 and V and L are,respectively, the flow of 13 and 14 streams.With this set of equation i could not solve this point of my problem since i had more unknows(F,V,L,X&Y for H2,X&Y for CO2) that equations(5 basic eqautions).
I can add another equation at my basic 5 since the CO2/H2 ratio given by text can be written as: Y(CO2)/Y(H2)=11.5 , but i still have 7 unknows vs 6 equtions, this is were i'm stuck.I'm searching for antoher equation or to find a way to obtain one of F,V or L.
At this point i omitted one another important thing, if you look at the picture there is the flow number 3 that is the exit from the primary reformer, i have every information of this flow(%compostion,total flow) but from this point until the flash i have no data, the problem jumps directly to the flash separator.

At this point my question cahnges,after the flow is passed through the secondary reformer, the two shift recators and the the column of sepration ,can i assume that all CH4 and CO from flow 3 becomes CO2 that is added to flow 12?


Finally, a small clarification, this is not a "homework" in the strict sense of the word,it is only an old exam wich i used(as the other that i have)for prepare myself to the september exam session.

[MrShorty]

There is something that i forgot to mention(my fault)in my post that was really important to solve the problem, i have the hernry constants for both H2 and CO2(given by text), then i already know the basic equations for the flash separator and are this one:
With this set of equation i could not solve this point of my problem since i had more unknows(F,V,L,X&Y for H2,X&Y for CO2) that equations(5 basic eqautions).
I can add another equation at my basic 5 since the CO2/H2 ratio given by text can be written as: Y(CO2)/Y(H2)=11.5 , but i still have 7 unknows vs 6 equtions, this is were i'm stuck.I'm searching for antoher equation or to find a way to obtain one of F,V or L.

It appears to me you have miscounted; I count 6 equations to go with the 7 unknowns:

henry law for the flash:

P_tot*Y_H2=H(H2)*X_H2 (1) same for CO2 (2)

global balances:

z*F=V*Y_H2+L*X_H2 (3) this formula can be also written for CO2 (4)

Y_H2+Y_CO2=1 (5)

F=V+L (6)

Where F is the total flow of stream 12 and V and L are,respectively, the flow of 13 and 14 streams.

Does that help?

One observation, you have no equations that describe H2O.

can i assume that all CH4 and CO from flow 3 becomes CO2 that is added to flow 12?

I'm not sure that you can, because you don't know what is going out the top of the separation column, do you?

[Luc1]

There is something that i forgot to mention(my fault)in my post that was really important to solve the problem, i have the hernry constants for both H2 and CO2(given by text), then i already know the basic equations for the flash separator and are this one:
With this set of equation i could not solve this point of my problem since i had more unknows(F,V,L,X&Y for H2,X&Y for CO2) that equations(5 basic eqautions).
I can add another equation at my basic 5 since the CO2/H2 ratio given by text can be written as: Y(CO2)/Y(H2)=11.5 , but i still have 7 unknows vs 6 equtions, this is were i'm stuck.I'm searching for antoher equation or to find a way to obtain one of F,V or L.

It appears to me you have miscounted; I count 6 equations to go with the 7 unknowns:

henry law for the flash:

P_tot*Y_H2=H(H2)*X_H2 (1) same for CO2 (2)

global balances:

z*F=V*Y_H2+L*X_H2 (3) this formula can be also written for CO2 (4)

Y_H2+Y_CO2=1 (5)

F=V+L (6)

Where F is the total flow of stream 12 and V and L are,respectively, the flow of 13 and 14 streams.

Does that help?

One observation, you have no equations that describe H2O.

can i assume that all CH4 and CO from flow 3 becomes CO2 that is added to flow 12?

I'm not sure that you can, because you don't know what is going out the top of the separation column, do you?

Yep you're right,i have 7 equations and 7 unknows it could be solved...but it cant cause from the set of equation of henry law combined with the two eqution Y+Y2=1 and Y/Y1=11.5 i can obtain the compostion of the liquid stream and the gas stream.At this point i would calculate F V and L but the system now can not be solved,cause i have only the composition of the streams,and the systems return with a value of zero for F V and L.

So i checked and discoverd that the gas that is pulled away from the clomun is a ammonia systesis gas which cant(or must not)contain CO or CO2, so I assume that: Whereas the process equipment operate optimally, the methane is completely transformed into a CO2 which is then removed in the column wash, and is sent to the flash separator.So can obtain the total flow of the 12 using a mass balance on CO2:

moles_CO2,produced=0.0037*F₁₂

I do not need equations on H2O as it is completely discharge into the liquid stream of flash.

[MrShorty]

I do not need equations on H2O as it is completely discharge into the liquid stream of flash.

Wouldn't this be equivalent to "I don't need equations for H2, because it is completely discharged into the vapor stream of the flash?" Even if you are making the assumption that there is no water in stream 13 (Can you use Raoult's law or similar to validate this assumption?), I think you want to formalize that assumption with the necessary equations to calculate the feed and liquid compositions for H2O as well.

[Luc1]

I do not need equations on H2O as it is completely discharge into the liquid stream of flash.

Wouldn't this be equivalent to "I don't need equations for H2, because it is completely discharged into the vapor stream of the flash?" Even if you are making the assumption that there is no water in stream 13 (Can you use Raoult's law or similar to validate this assumption?), I think you want to formalize that assumption with the necessary equations to calculate the feed and liquid compositions for H2O as well.

Let me justify what i said,if you look at the scheme that i have posted before you can see that next to the arrow of stream 13 is written "H2+CO2",now, as our knowledge about how a flash separator are as follows: "It is a machine that separates the liquid and the gaseous phase,completly, by a stream that contains both and works at a fixed temperature and pressure" ,it is logical for us to assume that the water, at least ideally, will not be present in the stream 13.In conclusion, I know that there should be water vapor in the flash gas stream (due to Rault's law, the real flash cant separate completly the 2 phases) but because the wiring diagram shows me there is none in the gas stream, I assume that the water is completely discharged in liquid phase.

Following this reasoning the H2O equation that applies to my flash is this one:

(1-0.0027-0.037)*F=L*x_H2O

i dont use this equation because i dont know how much water is added in the washing column, on the contrary i know how much CO2 is produced(The limiting reactant is the CH4)during the reforming route and is completly added to the stream 12.

If I was not clear in the earlier speech I will say it in simpler terms (I'm not able at times to make myself understood in Italian, how can I expect to do it in English?): basically the machines that form a plant are treated in a simplified manner for which various simplifications are made for us to understand in general how the plant operates(in this case, the steam reforming), later in other courses such simplifications will be deleted and we will study the actual function of each machine.

PS:I forgot to thank you for aswering to my questions.

Edited by Luc1, 11 September 2009 - 07:47 PM.