20 replies to this topic

[Venkat @89]

Hi everyone,


We have got Naphtha splitter that splits Naphtha into Light and Heavy Naphtha, whose column pressure is 1.5kgf/cm2g and top temperature is 105degC. Reflux drum pressure is maintained above 0.7kgf/cm2g.

Just want to understand the importance of maintaining pressure in reflux drum via Hot bypass PDV? What factor decides the minimum value of pressure to be maintained in it?

How does variation of pressure via Pressure differential valve in reflux drum affects the top product quality (Light Naphtha) or the process?. 

Attached Files

•  NS PFD.pdf   151.56KB   25 downloads

Edited by Venkat1989, 05 December 2022 - 06:50 AM.

[shvet1]

Just want to understand the importance of maintaining pressure in reflux drum via Hot bypass PDV? 

Depends on type of condenser.

If condenser is air cooled then PDV is a vacuum breaker. It protects reflux drum from collapsing during heavy rain.

If condenser is water cooled + partially flooded then PDV controls liquid level in tubesheet. It changes % of tubes flooded and by this way rate of heat transfer and resulting pressure in splitter.

See chap. 17 Kister's Distillation Operation.

 

Provide P&ID of splitter to comment more accurate. More details = better answers.

Edited by shvet1, 05 December 2022 - 02:12 AM.

[SilverShaded]

 

Just want to understand the importance of maintaining pressure in reflux drum via Hot bypass PDV? 

Depends on type of condenser.

If condenser is air cooled then PDV is a vacuum breaker. It protects reflux drum from collapsing during heavy rain.

If condenser is water cooled + partially flooded then PDV controls liquid level in tubesheet. It changes % of tubes flooded and by this way rate of heat transfer and resulting pressure in splitter.

See chap. 17 Kister's Distillation Operation.

 

Provide P&ID of splitter to comment more accurate. More details = better answers.

 

Wouldn't the drum be designed for full vacuum?

[Venkat @89]

@Shvet1

 

Attached the PFD, Kindly let me know, if reflux drum pressure variation with PDV has anything to do with top product quality?

Attached Files

•  NS PFD.pdf   151.56KB   20 downloads

Edited by Venkat1989, 05 December 2022 - 06:49 AM.

[SilverShaded]

@Shvet1

 

Attached the PFD, Kindly let me know, if reflux drum pressure variation with PDV has anything to do with top product quality?

Drum pressure does not affect top product quality, only cut point and reflux ratio will as the the condenser is sub-cooled.

[shvet1]

@Shvet1

 

Attached the PFD, Kindly let me know, if reflux drum pressure variation with PDV has anything to do with top product quality?

1/ Pressure in V-016 does not affect C-006 overhead product composition as it is subcooled in E-023 and may be in A-006.

 

For info

In theory pressure in V-016 is allowed to be bellow atm - C-006 is able to operate properly at vacuum in V-016. Slightly gauge pressure 0.8 barg in V-016 is intended (1) to prevent a risk of air to be sucked in V-016 and (2) to provide a reasonable pressure drop of control valve placed on vent to flare to prevent accumulation of non-condensable gases in V-016. PDIC control valve pressure drop equal 0.7 bar (10 psi) is related to control valve placed on line between E-023 and V-016 and is a typical value for industrial control valves having common design. Pressure in V-016 0.8 barg can be decomposed as: 0.1 barg plus 0.7 bar (superimposed backpressure in flare header and industry practice for control valves having common design respectively). 

 

2/ PIC and TIC controllers on C-006 overhead vapor line do affect overhead product composition.

 

3/ PDIC controller is designed to maintain min pressure drop of control valve placed between E-023 and V-016. 

 

Disclamer

This statement may not be true - right answer depends on actual elevation differences of A-006, E-023 and V-016. You should take a look at Process Design Documentation or Process Manual or ask Fluor or provide your own calculation based on actual equipment design&elevation, piping isometrics and I&C datasheets.

 

4/ If pressure drop of PDIC would decrease bellow some limit (for some reason) then pressure drop of control valve downstream of E-023 decreases bellow min Cv value and PIC controller starts to oscillate => C-006 starts to oscillate => composition of overhead product becomes unstable or off-spec. Actual limits depends on inputs mentioned in para. 3. Also see disclaimer in para. 3.

 

5/ As an option of para. 4 pipe between E-023 and V-016 will self-drain to V-016 and control valve on the same pipe will not be able to function at all. See disclaimer in para. 3.

 

Advice - study Kister's handbooks for description of fractionation control this kind of, it's pros and cons and troubleshooting:

- Henry Z. Kister Distillation Design

- Henry Z. Kister Distillation Operation

- Henry Z. Kister Distillation Troubleshooting

This topic is not so complicated as it may seem.

Edited by shvet1, 06 December 2022 - 04:10 AM.

[Venkat @89]

@ Shvet1 

 

Thank you very much for detailed answer. I could learn something through your post.

 

Finally, the pressure in V-016 when PDV valve opening is zero, can we say the pressure in reflux drum is purely because of Vapor pressure of the liquid in the reflux drum at the existing temperature?

[shvet1]

Finally, the pressure in V-016 when PDV valve opening is zero, can we say the pressure in reflux drum is purely because of Vapor pressure of the liquid in the reflux drum at the existing temperature?

 

I do not understand language. Try to rephrase in simple way.

 

This is naphtha splitter, right? Not LPG fractionator like debutanizer, right? This fact means pressure in V-016 0.8 barg is much more than light naphtha vapor pressure at 60°C. Again - light naphtha in V-016 is subcooled what means it's temperature is below boiling point.

 

Pressure in V-016 is maintained equal 0.8 barg:

- during start-up - by an external source of inert gas

- during operation - by accumulation of non-condensable gases e.g. hydrogen and methane

 

PDC valve is normally closed. This valve opens:

- when balance of non-condensable gases becomes negative or

- in case of overload of A-006 and E-023 caused by failure of control loops e.g. heavy rain

Edited by shvet1, 11 December 2022 - 11:11 PM.

[SilverShaded]

There are no non-condensible gases in the drum.  The vapour is maintained by the bypass and the vapour is not in equilibrium with the liquid, that is the only reason it is two phase.  The bypass valve is there to ensure the vapour going to the air-cooler is the correct pressure to enable flow through the air-cooler and trim cooler.

Both the air-cooler and the condenser drum should be designed for full vacuum, if theyre not then somebody did a very poor HAZOP study on them...

[shvet1]

The bypass valve is there to ensure the vapour going to the air-cooler is the correct pressure to enable flow through the air-cooler and trim cooler.

Condensing equipment is driven by gravitation, not pressure drop.

 

Both the air-cooler and the condenser drum should be designed for full vacuum.

Disputable statement, many engineers would not agree.

Edited by shvet1, 12 December 2022 - 04:26 AM.

[SilverShaded]

 

The bypass valve is there to ensure the vapour going to the air-cooler is the correct pressure to enable flow through the air-cooler and trim cooler.

Condensing equipment is driven by gravitation, not pressure drop.

 

Both the air-cooler and the condenser drum should be designed for full vacuum.

Disputable statement, many engineers would not agree.

 

Not entirely, the pressure in the drum is controlled by the amount of vapour allowed through the bypass valve, the warmer vapour is not in equilibrium with the liquid in the drum, if it was there would be no vapour present.  It has nothing to do with non-condensibles which you suggested. 

In your scenario what happnes if the byapass valve is fully open, and drum pressure equals column top pressure?

[shvet1]

 

In your scenario what happnes if the byapass valve is fully open, and drum pressure equals column top pressure?

 

 

In case of uncontrolled rise of pressure in V-016: A-006 and V-016 will become overflood, pressure in C-006 will rise and PIC controller will open vent to flare.

 

Pressure rise in V-016 caused by accumulation of non-condensable gases (positive balance) to be controlled manually: measured by % open of DPIC and PIC control valves and maintained by vent control valve. Fluor expects that balance of non-condensable gases is negative and pressure in V-016 equals naphtha vapors pressure @ 60°C so non-equilibrium naphtha vapors to go to V-016 through DPIC valve permanently and to be absorbed by liquid phase.

Edited by shvet1, 12 December 2022 - 11:11 PM.

[SilverShaded]

 

 

In your scenario what happnes if the byapass valve is fully open, and drum pressure equals column top pressure?

 

 

In case of uncontrolled rise of pressure in V-016: A-006 and V-016 will become overflood, pressure in C-006 will rise and PIC controller will open vent to flare.

 

Pressure rise in V-016 caused by accumulation of non-condensable gases (positive balance) to be controlled manually: measured by % open of DPIC and PIC control valves and maintained by vent control valve. Fluor expects that balance of non-condensable gases is negative and pressure in V-016 equals naphtha vapors pressure @ 60°C so non-equilibrium naphtha vapors to go to V-016 through DPIC valve permanently and to be absorbed by liquid phase.

 

In the case of a typical physical layout for hot vapour by-pass do you still assert that the pressure drop is not driving the material through the cooler?

Attached Files

•  053.png   43.01KB   1 downloads

[breizh]

Hi ,

To add to your discussion the doc attached.

Schematic 11

Breizh

[shvet1]

In the case of a typical physical layout for hot vapour by-pass do you still assert that the pressure drop is not driving the material through the cooler?

I do not understand your point as text and picture show different situations. Try to rephrase in straight way, "child language" is preferred. As I have been taught - you understand a topic only when you are able to explain that to a child.

 

1/ Note that pressure drop on picture drives liquid through upward pipe segment marked as "H", not condenser. Condensing process can not be driven by pressure drop because of physic laws. Condenser on your picture is driven by gravity while pressure controller maintains condensing/sub-cooling surface ratio and so way overall heat-transfer coefficient.

 

2/ "driving the material through the cooler" - condenser, cooler or sub-cooler? You posted a picture with condenser, topicstarter posted PFD with 2 condensers in series. I do not understand what you are writing about.

 

3/ Pressure drop can drive whatever it was designed for (condenser or upward flow or control valve) and all options will be operable or not depending on combination of details. The answer depends on elevation of equipment and control valves specifications and only topicstarter has access to those.

 

4/ Note that pressure drop between A-006 and V-006 is 0.7 barg what equals ~10 m of naphtha column. It is not a typical design to elevate reflux drum up to 10 m above ground, right?

 

5/ Note that pressure in reflux drum on picture you posted is uncontrolled (as reflux vapor pressure depends on rate of sub-cooling in condenser) while on picture topicstarter posted it is controlled by DPIC controller.

 

6/ I have checked Design Documentation of 6 Refinery Process Units submitted by several reputable Licensors I have been involved in in the past having in total >12 naphtha splitters and fractionators with overhead naphtha total condensers. No one of those had fractionation control kind of posted by you. Should I name that "typical" after all of that? If I have never met it before is it typical for all around the world? This design is typical particular in your region/industry? As you can see what is obvious to you may not be the same to the others.

 

For info

2/3 of condensers had control this kind of

1/3 of condensers had control this kind of

1 condenser had control this kind of

Edited by shvet1, 14 December 2022 - 04:08 AM.

[SilverShaded]

The attached drawing shows typical layout for control purposes, the flow is not driven by cooling/condensation but by pressure drop.  I thnk we are probably saying similar things in a different way.  Condenser duty is obvously a function of heat transfer.  The pressure drop across the exchanger is what drives the material in the correct direction.

 

The attached diagram is very typical of what i see on stabilisers, with the exception that the flow control is typically on the product line and the level control on the reflux.

 ZI-0UIW-2004-SEP00-IDSI-33-1.jpg   38.04KB   2 downloads

Edited by SilverShaded, 31 December 2022 - 03:21 AM.

[breizh]

Hi Silvershaded,

Can you share with us the reference of your document?

Another article to consider is attached .

Thanks and regards

Breizh

[Divid Kelin]

I am working with total condenser in naphtha abs column
At the top of naphtha abs vapor line there is one PC controlling two pressure control valve one is bypass and another located at downstream of condenser.
Which is the better for controlling ?
PC at the vapor line controlling as mentioned above or
With this PC controlling bypass and upstream pressure control valve of the condenser or controlling downstream pressure control valve of the condenser and pressure control valve located at reflux drum? Bypass will be with One PIC.
Need your advice

Edited by Divid Kelin, 01 January 2023 - 10:53 AM.

[breizh]

Hi,

Better to start a new thread.

A good habit is to supply a PID to get a proper answer.

Breizh

[shvet1]

Which is the better for controlling ?
PC at the vapor line controlling as mentioned above or
With this PC controlling bypass and upstream pressure control valve of the condenser or controlling downstream pressure control valve of the condenser and pressure control valve located at reflux drum? Bypass will be with One PIC.
Need your advice

 

There is no one simple answer. Optimum option depends on combination of details. 

 

Advice - study Kister's handbooks for description of fractionation control this kind of, it's pros and cons and troubleshooting:

- Henry Z. Kister Distillation Design

- Henry Z. Kister Distillation Operation

- Henry Z. Kister Distillation Troubleshooting

This topic is not so complicated as it may seem.

 

You should provide us with much more info to help you.

[Venkat @89]

Thanks everyone for giving your valuable inputs and made me to learn few new things behind this.