2 replies to this topic

[process705]

Hi Every One:
Why do we give extra space for liquid in Contactor? when we keep our liquid level allot less then the space usually we have in bottom of the amine Contactor. On top of the Contactor we give extra space for vapors or gas to accumulated. but in bottom????
i do understand that we need to maintain some liquid level there.

[Art Montemayor]

Process:

This is a basic, but good question coming from a student and opens the door for some key learnings.

I don’t know what you mean by “extra space”. The sump (bottom section of a tower) should always be engineered as to size – both in volume and height. This is an important item when one considers the practical, real-life operation of the tower itself.

If you have a “contactor” (an amine absorber) with packing or with trays, you will have liquid amine solution within the trays and packing while the tower is in operation – more so, in the case of trays. The moment the tower shuts down due to a process upset, an emergency, or a normal shutdown, you will have liquid amine solution “raining” down the tower and into the sump. It therefore is common sense to allow for sufficient liquid inventory in the sump in order to avoid – guess what? – an overflow of amine solution back into the upstream gas inlet line that feeds the bottom of the tower. This can not only cause a lot of problems, it could also be a jeopardy to safe operations during the shutdown and in the subsequent startup.

Additionally, every closed loop operation – which an amine unit is – requires sufficient internal volume in key equipment to allow for solution storage – especially during shutdowns and startups. Again, common sense dictates that you have to have a place to put the stuff in when you are not running the unit. This extra process volume is usually allowed for in a kettle reboiler (if you use this type) or in the sump of the absorber. Sometimes you have a flash drum and a surge vessel which also serve as potential reservoirs.

I don’t know where you read it or who told you that “On top of the Contactor we give extra space for vapors or gas to accumulated”, but that is false and wrong information. You allow for disengagement space above the last point where liquid and gas are in physical contact in any tower or vessel because it is required to allow both phases to separate without causing an entrainment of one phase (usually the liquid) to exit with the gas. If you are studying Unit Operations, then I should seriously remind you that you are supposed to know about the Brown-Souders relationship – which is used to determine the distance between trays and the size of a liquid-vapor separator.

You are required to have a liquid level in the sump of the tower in order to have a seal and be able to control the existence of a gas phase in the bottom section. Read one of my recent posts on this very subject and you will further understand the significance of this and why it is a practical necessity in Unit Operations and process controls.

I hope these explanations help you out.

[Zauberberg]

And get a copy of GPSA Databook; you'll find there plenty of information with regard to sizing of tower sumps. The residence time required varies upon the configuration of facilities downstream of the tower (storage or another plant), and the way how they are being fed (level control, flow control, pumping, etc.)