17 replies to this topic

[osigikim]

Good morning.
I'd like to simulate Deethanizer System by HYSYS to predict the C2 product composition and flow rate out of column overhead.
The specified values for column simulation are as followings:
 1. reflux ratio : 0.86
 2. condenser duty : -1222kW
And number of stages is 34.

Also I put values for bottom reboiler heat and overhead condenser heat like below:    
 1. bottom reboiler heat : 1607kW
 2. overhead condenser heat : -1222kw (as per above condenser duty)

My understanding is that according to changing of Heat Balance(reboiler heat and condenser heat), Material Balance is changing accordingly. So I think that from above conditions, I could get information about vent, C2 product and Bottom product. But I can not do simulation with above conditions.

Please advise me with your kind comments about what I missed and I should follow.

I am sorry to say I can't upload simulation file, so kindly give me email address if you need for your review.

Your kind attention and comments would be very much appreciated.

Edited by osigikim, 18 March 2017 - 10:25 PM.

[PingPong]

What you are trying to do seems nonsense to me. This is not the way to simulate a Deethanizer, or any other distillation column for that matter.

 

Is this an existing column, or are you designing a new Deethanizer?

 

In what kind of process unit is this Deethanizer located? In a refinery? Or an ethylene plant? Or what?

 

If it is not confidential, what is the feed composition?

[Bobby Strain]

You should first read the HYSYS manual. There you will find how to properly simulate distillation.

 

Bobby

[osigikim]

Dear Mr.Pingpong

Good morning. gentleman.
Thank you very much for your kind attention.
Please find the information below for your reference.

This is existing column.
This column is in LNG liquifaction plant.

Feed gas composition[mol%]
N2(0.007), C1(9.793), C2(8.015), C3(14.163), iC4(8.036), nC4(15.727), iC5(15.669), nC5(11.397), nC6(7.942), C7+(7.350), Benzene(0.474), Toluene(1.423), CO2(0.002), H2S(0.001)

Feed gas flow rate : 273.2[kgmol/h]

overhead top product spec : ethane 79% & 6.1[kgmol/h] liquid
overhead vent : ethane 40% & 42.7[kgmol/h] vapor
Bottom product spec : ethane 0.045% & 224.4[kgmol/h] liquid

In my understanding, according to above conditions, existing column is designed including column pressure & top temperature & bottom temperature & reflux flow rate & column stage number.
Top temperature is controlled by condenser which heat flow is -1222kW.
Bottom temperature is controlled by reboiler which heat flow is 1607kW.   

But currently, overhead top product is not within specification. ethane composition is now around 72~74%.

So I'd like to know root cause about this problem by simulating HYSYS.

But at present I can not simulate HYSYS with above information.
For example, I'd check bottom temperature is too high or not etc.
In this case, in my understanding by increasing reboiler heating flow, I can get the new overhead C2 composition in new condition.

 

Please advise me about this.

 

Best regards,

osigikim

[cea]

Dear osigikim,

 

I do not know what is your competency for HYSYS simulation.

 

I will suggest you to first identify the input parameters to HYSYS simulation & play with that to get desired goal. You cannot play with results & expect to get other parameters adjusted automatically, unless you are expert in using "Adjust" function.

[osigikim]

Dear

 

Dear osigikim,

 

I do not know what is your competency for HYSYS simulation.

 

I will suggest you to first identify the input parameters to HYSYS simulation & play with that to get desired goal. You cannot play with results & expect to get other parameters adjusted automatically, unless you are expert in using "Adjust" function.

Dear Mr. Cea,

Thank you very much for your kind attention.

I am just beginner for HYSYS, so I have difficulty for simulating deethanizer as you said.

Could you please give me your email address for me to send my file for your kind review?

It would be very appreciated for you to review my file.

 

Best regards,

[osigikim]

You should first read the HYSYS manual. There you will find how to properly simulate distillation.

 

Bobby

Dear Mr. Strain,

Thank you very much for your kind attention.

I am just beginner for HYSYS. I am trying to practice case by case.

But it's hard to simulate distillation column.

Please advise any case study for distillation column using HYSYS if any.

Your kind attention is highly appreciated.

 

Best regards,

[PingPong]

A Deethanizer makes a separation between a top product of C2 (and lighter) and a bottom product of C3 (and heavier).

To simulate such a column you need to specify the C3 content of the top product and the C2 content of the bottom product.

Bottom product spec : ethane 0.045%

So you know already the C2 spec to be used for the bottom product. Question is only: what does % mean in this case? Mol or weight or volume?

That value of 0.045 % sounds very low though. Must it really be that low?

 

Now you only need to figure out what the C3 in the ethane top product is supposed to be. How much % C3 is in the C2 product during present operation? And how much should it really be to make everybody happy? What is this C2 product to be used for?

 

You also need to specify the condensor temperature, and of course the condensor pressure and column pressure top to bottom.

 

The condensor T and p determine how much overhead vent you will get and how much C2 you lose with that. It also determines how much lights dissolve in the liquid ethane top product. These dissolved lights plus some C3 reduce the purity of the liquid ethane top product. You say it is now around 72~74% (mol or weight or volume % ?) but you should really have the lab determine how much C3 is in it.

 

By increasing the condensor temperature you could increase the liquid ethane top product purity by reducing dissolved lights in it, but in that way you would lose more ethane with the vent. Increasing the ethane product purity is then best done by operating the upstream demethanizer such that it leaves less lights in the deethanizer feed. Note that the feed now contains quite a lot of C1.

 

Let's not worry too much about the ethane product purity right now.

If you don't know the actual C3 spec for the ethane product then just assume something realistic to get the simulation going. Take for example 0.5 mol% to start with and worry about the correct value later.

For the bottom product specify that 0.045 % C2, or a higher value if allowed.

Specify condensor T and P.

Specify pressure profile from condensor to column bottom.

Specify number of stages.

Select a thermodynamic model.

And that's it really.

Maybe it will help to add some estimates of flowrates, tray loadings, duties. As I never use Hysys, but often another process simulator, I don't know what it takes to get it converged in Hysys.

 

Could you tell us what the T and P of the condensor are, as well as the T and P of the feed?

You mentioned before that number of stages is 34. Is that theoretical stages or actual? And is that including condensor and reboiler, or not?

[Bobby Strain]

If you have a license for HYSYS, you should have the users guide. And, there is lots of information, including examples, at the AspenTech support site. You will get much better information through HYSYS resources than you will get here. So you need to use those resources.

 

Bobby

[osigikim]

A Deethanizer makes a separation between a top product of C2 (and lighter) and a bottom product of C3 (and heavier).

To simulate such a column you need to specify the C3 content of the top product and the C2 content of the bottom product.

 

 

Bottom product spec : ethane 0.045%

So you know already the C2 spec to be used for the bottom product. Question is only: what does % mean in this case? Mol or weight or volume?

That value of 0.045 % sounds very low though. Must it really be that low?

 

Now you only need to figure out what the C3 in the ethane top product is supposed to be. How much % C3 is in the C2 product during present operation? And how much should it really be to make everybody happy? What is this C2 product to be used for?

 

You also need to specify the condensor temperature, and of course the condensor pressure and column pressure top to bottom.

 

The condensor T and p determine how much overhead vent you will get and how much C2 you lose with that. It also determines how much lights dissolve in the liquid ethane top product. These dissolved lights plus some C3 reduce the purity of the liquid ethane top product. You say it is now around 72~74% (mol or weight or volume % ?) but you should really have the lab determine how much C3 is in it.

 

By increasing the condensor temperature you could increase the liquid ethane top product purity by reducing dissolved lights in it, but in that way you would lose more ethane with the vent. Increasing the ethane product purity is then best done by operating the upstream demethanizer such that it leaves less lights in the deethanizer feed. Note that the feed now contains quite a lot of C1.

 

Let's not worry too much about the ethane product purity right now.

If you don't know the actual C3 spec for the ethane product then just assume something realistic to get the simulation going. Take for example 0.5 mol% to start with and worry about the correct value later.

For the bottom product specify that 0.045 % C2, or a higher value if allowed.

Specify condensor T and P.

Specify pressure profile from condensor to column bottom.

Specify number of stages.

Select a thermodynamic model.

And that's it really.

Maybe it will help to add some estimates of flowrates, tray loadings, duties. As I never use Hysys, but often another process simulator, I don't know what it takes to get it converged in Hysys.

 

Could you tell us what the T and P of the condensor are, as well as the T and P of the feed?

You mentioned before that number of stages is 34. Is that theoretical stages or actual? And is that including condensor and reboiler, or not?

 

Dear Pingpong,

 

Thank you very much for your kind comments.

Below are some information for your reference.

 

% is mol %.
T and P for feed is 110degC and 47.47kg/cm2g respectively.
Number of stage for this column is 34 which is actual not theoretical except condenser or reboiler.
Outlet from condenser is -33.1degC, 25.68kg/cm2g.

 

Please let me know if you need information.

 

Best regards,

[osigikim]

If you have a license for HYSYS, you should have the users guide. And, there is lots of information, including examples, at the AspenTech support site. You will get much better information through HYSYS resources than you will get here. So you need to use those resources.

 

Bobby

Dear Bobby,

 

Thank you very much for your kind comments.

As you said, I would find the users guide.

 

Best regards,

[PingPong]

What is the actual operating bottoms temperature of the Deethanizer column?

 

You earlier mentioned a gas feed composition and flowrate, but you also mentioned that the feed is at 110 ^oC and 47.47 kg/cm²g which will give an only partly vaporized feed at the column inlet, only about 20 mol% vapor. Why do you think the deethanizer feed is gas?

 

You earlier mentioned a reflux ratio of only 0.86 but when I do a simulation using PRO/II it is about 8.5 so I assume you made a typing error and you meant 8.6 i/o 0.86 ?

 

To obtain the duties you mentioned before I had to use a tray efficiency of only 50 %, which seems very low for this service.

 

My simulation gave a 79 % purity of the liquid ethane product, based on an assumed C3 content of 0.1 kmol/h.

 

I have the impression that all the flowrates and compositions you supplied are theoretical, from the design material balance of the unit, or somebody else's simulation, not from actual operation measurements. Am I correct?

[osigikim]

What is the actual operating bottoms temperature of the Deethanizer column?

 

You earlier mentioned a gas feed composition and flowrate, but you also mentioned that the feed is at 110 ^oC and 47.47 kg/cm²g which will give an only partly vaporized feed at the column inlet, only about 20 mol% vapor. Why do you think the deethanizer feed is gas?

 

You earlier mentioned a reflux ratio of only 0.86 but when I do a simulation using PRO/II it is about 8.5 so I assume you made a typing error and you meant 8.6 i/o 0.86 ?

 

To obtain the duties you mentioned before I had to use a tray efficiency of only 50 %, which seems very low for this service.

 

My simulation gave a 79 % purity of the liquid ethane product, based on an assumed C3 content of 0.1 kmol/h.

 

I have the impression that all the flowrates and compositions you supplied are theoretical, from the design material balance of the unit, or somebody else's simulation, not from actual operation measurements. Am I correct?

Dear Pingpong,

 

Those are design parameters.

 

Feed is not gas but liquid.

Actual operating bottom temperature is 150degC (design:147degC)

Actual operating outlet from condenser is -32degC (degisn:-33.1degC)

I'd like to simulate deethanizer using design parameters first and then check the composition difference by changing bottom temperature and condenser temperature.  

Under current operating conditions, if top product quality(C2 composition) is not met within expectation, I'd like to consider to increase operating pressure in column to catch C2 vapor more to increase C2 composition in top distillate.

 

Your kind comments would be very much appreciated.

 

Best regards,

[PingPong]

If you want to know what the composition is in actual operation then you should have the laboratory do an analysis of vent gas, liquid distillate and bottoms.

 

The fact that present condensor and bottoms temperatures differs a little from original design does not necessary mean that C3 content in ethane or C2 content in bottoms are very different.

The present feed compostion is very likely different from the original design. That will have some impact on the botoms temperature even if the C2 content is the same as in the original design.

Moreover the C7⁺ part of the feed is not detailed so doing an accurate simulation of actual operation bottoms temperature is not possible anyway. When I did my simulation and used my thumb to break it down in multiple fractions. As it happens my simulation gave the 147 ^oC bottoms temperature of the original design, but that is partly luck and partly experience with precision guesswork.

 

The lower-than-design condensor temperature will dissolve more lights into the liquid ethane distillate, and increasing it would increase the ethane purity, but it would also decrease the liquid ethane yield.

I mentioned that already in post #8 and suggested to adjust the operation of the upstream demethanizer to lower the C1 content of the deethanizer feed.

Or is there no upstream demethanizer?

 

In any case I find the obsession with that small quantity of liquid ethane purity very strange.

About 80 % of the ethane from the feed ends up in the vent gas, only 20 % is recovered as liquid ethane.

What is this liquid ethane product to be used for?

And what happens with the vent gas?

Edited by PingPong, 22 March 2017 - 08:03 AM.

[osigikim]

If you want to know what the composition is in actual operation then you should have the laboratory do an analysis of vent gas, liquid distillate and bottoms.

 

The fact that present condensor and bottoms temperatures differs a little from original design does not necessary mean that C3 content in ethane or C2 content in bottoms are very different.

The present feed compostion is very likely different from the original design. That will have some impact on the botoms temperature even if the C2 content is the same as in the original design.

Moreover the C7⁺ part of the feed is not detailed so doing an accurate simulation of actual operation bottoms temperature is not possible anyway. When I did my simulation and used my thumb to break it down in multiple fractions. As it happens my simulation gave the 147 ^oC bottoms temperature of the original design, but that is partly luck and partly experience with precision guesswork.

 

The lower-than-design condensor temperature will dissolve more lights into the liquid ethane distillate, and increasing it would increase the ethane purity, but it would also decrease the liquid ethane yield.

I mentioned that already in post #8 and suggested to adjust the operation of the upstream demethanizer to lower the C1 content of the deethanizer feed.

Or is there no upstream demethanizer?

 

In any case I find the obsession with that small quantity of liquid ethane purity very strange.

About 80 % of the ethane from the feed ends up in the vent gas, only 20 % is recovered as liquid ethane.

What is this liquid ethane product to be used for?

And what happens with the vent gas?

Dear Pingpong,

There is scrub column upstream De-C2 as function for light component catcher.

Distillate from De-C2 is used for component for MR(mixed refrigerant).

 

I found that by increasing bottom temperature, C2 purity in distillate is increasing in HYSYS.

And by decreasing condenser temperature, C2 purity in distillate is increasing HYSYS.

In this finding, I am understanding that C2 is vaporized more in the De-C2 due to high bottom temperature and C2 is condensed more due to low condenser temperature. Is my understanding correct?

In current operation condition, I can not change upstream De-C2 and condensate temperature. So the only option is increasing bottom temperature.

 

Your kind comments would be very much appreciated.

Best regards,

[PingPong]

I found that by increasing bottom temperature, C2 purity in distillate is increasing in HYSYS.

And by decreasing condenser temperature, C2 purity in distillate is increasing HYSYS.

A first sight that does not make sense to me.

 

But I would need to see your results to understand what actually changed between the simulations.

 

Could you make a table in Excel for:

- your original simulation (base case),

- the simulation with increased bottoms temperature,

- the simulation with decreased condensor temperature

summarizing the following information:

- condensor T and P

- condensor duty

- vent gas molar quantity and molar composition

- liquid ethane distillate molar quantity and molar composition

- reflux molar quantity

- bottoms product molar quantity and molar composition

- bottoms T and P

- reboiler duty

[osigikim]

 

I found that by increasing bottom temperature, C2 purity in distillate is increasing in HYSYS.

And by decreasing condenser temperature, C2 purity in distillate is increasing HYSYS.

A first sight that does not make sense to me.

 

But I would need to see your results to understand what actually changed between the simulations.

 

Could you make a table in Excel for:

- your original simulation (base case),

- the simulation with increased bottoms temperature,

- the simulation with decreased condensor temperature

summarizing the following information:

- condensor T and P

- condensor duty

- vent gas molar quantity and molar composition

- liquid ethane distillate molar quantity and molar composition

- reflux molar quantity

- bottoms product molar quantity and molar composition

- bottoms T and P

- reboiler duty

 

Dear Pingpong,

 

Design parameters for HYSYS is as followings:

Reflux Ratio is 8.0

Distillate Rate is 6.1 kgmol/h

Bottom Product Rate is 224.4 kgmol/h

 

And HYSYS result is attached in excel table.

As per my second review, it seems like that distillate purity is mainly affected condenser condition.

By decreasing condenser temperature, distillate purity is improved.

 

And I have another idea for improving C2 purity.

Is it helpful to install "mist seperator" in reflux drum to catch more C2 vapor to prevent C2 vapor from venting?

 

Your kind comments would be very much appreciated.

Best regards, 

Attached Files

•  question on De-c2 simulation.xlsx   19.46KB   20 downloads

[PingPong]

You have been cheating in those simulations.

 

When you decreased the condensor temperature you also decreased the condensor pressure. It is the drop in condensor pressure that gives a small improvement in ethane purity, not the lower temperature. In fact the lower temperature partly works against the positive effect of lower condensor pressure.

 

And when you changed the bottoms temperature also the bottoms pressure was changed, while keeping the condensor pressure the same. But it had no noticeable effect on ethane purity anyway.

Increasing bottoms temperature while maintaining the same column pressure will give a slight decrease in ethane purity because you will get a little more C3 in the ethane distillate and every extra mol C3 will absorb more C1 than a mol of C2. So ethane purity slightly drops due to more C3 and C1 present.

 

You cannot improve the ethane product purity by increasing the bottoms temperature.

You cannot improve the ethane product purity by lowering the condensor temperature. To the contrary: a lower temperature results in more C1 dissolving in the ethane product and consequently a lower ethane purity. To improve ethane purity you need to lower its C1 content by increasing the condensor temperature. But that will also increase the ethane loss in the vent gas, unless you first lower the C1 content in the deethanizer feed.

 

The root cause of the lower-than-design ethane product purity is a higher-than-design C1 content of the deethanizer feed.

The way to improve the ethane product purity is to reduce the amount of C1 in the deethanizer feed by improving the operation of the upstream scrub column. And after that you can increase the ethane product purity by increasing (not decreasing) the deethanizer condensor temperature so that less C1 dissolves in the ethane product. In addition to that decreasing the pressure in the condensor will also help a little to improve the ethane product purity as that will also lower the C1 content in the ethane.

 

 

And I have another idea for improving C2 purity.

Is it helpful to install "mist seperator" in reflux drum to catch more C2 vapor to prevent C2 vapor from venting?

No. A mist eliminator will not catch C2 vapor but entrained liquid, which has exactly the same composition as the liquid ethane product from the same reflux drum. It might have a very small impact on ethane product recovery but has absolutely no effect on its purity.

Edited by PingPong, 24 March 2017 - 06:20 AM.