4 replies to this topic

[aliadnan]

Hi
While going through some text about heat exchange and column problems I come to know that the pressure of Fractionation Column increases if the Reflux rate is increased (too high). The pressure also increases if the condensor is overloaded with vapors due to condensor fouling.
Well I am not able to understand successfully that how this happens. I request the Experienced Engrs to help me in understanding this.

There is another question I have also read that when the condensor is fouled the water side pressure increases. Please explain it also

Thanking you in advance for help

Regards
Ali

[aliadnan]

HI Mr. Beychok & Mr. Montemayor can you help me in this regard. I am still waiting for replies from the experinced people who can help me in understanding this

[Art Montemayor]

Ali:

There are many things that can cause a given distillation operation to increase in operating pressure – and that pressure increase can be measured at more than one place. It all depends on the mechanical design, the feed conditions, the condenser operation, the reboiler’s operation, the reflux ratio, etc. etc..

I believe you are struggling with understanding and conceptualizing what is happening within a distillation column because you are worried about identifying what can cause you problems – when you should be worried about understanding what fixes the operating pressure within the column. What can go wrong can be better understood when you have a complete and detailed understanding about what you are causing within the column and why.

Under normal, usual conditions it is the vapor pressure of the liquid on the top tray that fixes the pressure at that location – directly before the generated vapors exit to enter the overheads condenser. This is the basic parameter that fixes the column pressure. The pressure at other sites within the column depends on the ability of the vapors and liquids to distribute themselves up and down the column with a minimum of pressure drop. Therefore, the composition – or purity – of the liquid on the top tray pretty well defines what pressure the column is expected – or designed – to operate at. The external reflux ratio (L/D) has a bearing on fixing that composition – as the various L/V’s (internal reflux ratios) that are generated down the column have on the various trays’ compositions.

Any distillation column has a fixed diameter – and due to this characteristic, has a maximum L/V that it can tolerate before excessive vapor resistances are generated and cause an increased pressure drop (translates to an increased column pressure build-up). This is common sense.

Additionally, if there are non-condensables built-up (accumulated) within the column due to the Feed liquid being contaminated or some thing else, then you will generate an excessive column pressure because of the inability to condense some of the vapors – an effect that is not usually engineered into the design of the column’s operation. This should also be common-sensical.

If you are unable to generate the design operation (and required L/D – external reflux ratio) because your overheads condenser is not operating efficiently for whatever reason, then you are not operating the column in compliance with the design and the pressure inside the column will increase due to your inability to furnish an overheads condenser capacity. This usually forces you to reduce the column capacity by reducing the reboiler's duty. Everything seems to “snowball” at this stage. Something is wrong with your reasoning if you don’t understand that you must operate the column at the conditions for which it was designed. Usually there is little margin for deviating from what you must furnish in order to accomplish a given separation. You MUST furnish efficient heat transfer rates in your condenser as well as in your reboiler. Note that I say “heat transfer rates” – and not merely heat transfer areas. The areas mean nothing if they are not operating efficiently. If you allow the water side of a condenser to foul with such undesirables as calcium carbonate build-up, you will practically plug the water side to a point where the water flow through becomes almost non-existent. At that point, you will note that the pressure drop increases astronomically for the water flow – to the point where the water flow ceases.

The best – and only way – to understand the operations of a distillation column is to carefully detail out the step-by-step methodology of designing the column. Then you should have the ability to conceptualize what is happening within the column when you introduce liquid and vapor in counter-current flow through a packed bed or through a series of trays. By knowing that you must furnish reboil heat to drive the column and also furnish condenser duty to furnish reflux, then you can understand what is happening and go forth to analyze undesirable effects that take place while the column is under operation. The only way to trouble-shoot a distillation column’s bad operation is to dominate the design parameters and the detailed workings of each of its components.

The degree to which you are successful in trouble-shooting a column is determined in large part by your field experience and hands-on skills.

I hope this helps you understand some of the factors that seem to be giving you problems.

[Viney]

I want to convey my sincere thanks, you explained the whole thing so nicely..... it helped me clear all the doubts i had regarding this topic.. thank again

Ali:

There are many things that can cause a given distillation operation to increase in operating pressure – and that pressure increase can be measured at more than one place. It all depends on the mechanical design, the feed conditions, the condenser operation, the reboiler’s operation, the reflux ratio, etc. etc..

I believe you are struggling with understanding and conceptualizing what is happening within a distillation column because you are worried about identifying what can cause you problems – when you should be worried about understanding what fixes the operating pressure within the column. What can go wrong can be better understood when you have a complete and detailed understanding about what you are causing within the column and why.

Under normal, usual conditions it is the vapor pressure of the liquid on the top tray that fixes the pressure at that location – directly before the generated vapors exit to enter the overheads condenser. This is the basic parameter that fixes the column pressure. The pressure at other sites within the column depends on the ability of the vapors and liquids to distribute themselves up and down the column with a minimum of pressure drop. Therefore, the composition – or purity – of the liquid on the top tray pretty well defines what pressure the column is expected – or designed – to operate at. The external reflux ratio (L/D) has a bearing on fixing that composition – as the various L/V’s (internal reflux ratios) that are generated down the column have on the various trays’ compositions.

Any distillation column has a fixed diameter – and due to this characteristic, has a maximum L/V that it can tolerate before excessive vapor resistances are generated and cause an increased pressure drop (translates to an increased column pressure build-up). This is common sense.

Additionally, if there are non-condensables built-up (accumulated) within the column due to the Feed liquid being contaminated or some thing else, then you will generate an excessive column pressure because of the inability to condense some of the vapors – an effect that is not usually engineered into the design of the column’s operation. This should also be common-sensical.

If you are unable to generate the design operation (and required L/D – external reflux ratio) because your overheads condenser is not operating efficiently for whatever reason, then you are not operating the column in compliance with the design and the pressure inside the column will increase due to your inability to furnish an overheads condenser capacity. This usually forces you to reduce the column capacity by reducing the reboiler's duty. Everything seems to “snowball” at this stage. Something is wrong with your reasoning if you don’t understand that you must operate the column at the conditions for which it was designed. Usually there is little margin for deviating from what you must furnish in order to accomplish a given separation. You MUST furnish efficient heat transfer rates in your condenser as well as in your reboiler. Note that I say “heat transfer rates” – and not merely heat transfer areas. The areas mean nothing if they are not operating efficiently. If you allow the water side of a condenser to foul with such undesirables as calcium carbonate build-up, you will practically plug the water side to a point where the water flow through becomes almost non-existent. At that point, you will note that the pressure drop increases astronomically for the water flow – to the point where the water flow ceases.

The best – and only way – to understand the operations of a distillation column is to carefully detail out the step-by-step methodology of designing the column. Then you should have the ability to conceptualize what is happening within the column when you introduce liquid and vapor in counter-current flow through a packed bed or through a series of trays. By knowing that you must furnish reboil heat to drive the column and also furnish condenser duty to furnish reflux, then you can understand what is happening and go forth to analyze undesirable effects that take place while the column is under operation. The only way to trouble-shoot a distillation column’s bad operation is to dominate the design parameters and the detailed workings of each of its components.

The degree to which you are successful in trouble-shooting a column is determined in large part by your field experience and hands-on skills.

I hope this helps you understand some of the factors that seem to be giving you problems.

[Qalander (Chem)]

Dear aliadnan Now you have got a thorough,comprehensive and analytical guidance indeed from our Art.

Now you may attempt actions on the identified items to explore the actuals

then rectify/ troubleshoot one item at one time with system's response observation

then fine tuning or further steps. You should be able to sort out successfully.
In your case you may start from

feed having any non-condensibles or

from the condenser's Cooling Water side for fouling.