Hello everyone well I am a bit confused in falling film evaporators and special reactor designs in which product gases leave from the bottom of the reactor instead from the top although it is a well known fact that gases always rise due to low density.Similar is the case with the falling film evaporators.Please can you tell that if it is due to distributor at the top which forces feed gases downward with a high dynamic pressure which forces the product gases to leave from the bottom instead from the top.Please I need some help in this regard from qualified engineers who are the surfers of this forum.

Before I answer this query, I would draw your attention to the awkward and disruptive manner you have intentionally joined your sentences together without the customary (& often required) spacing between them in order to facilitate the reader to understand your written thoughts. What you have probably accomplished with this foolish and disruptive act is to make it yet more difficult for those “qualified engineers … of this forum” to make the time and effort to respond to your request. That is a very foolish and inane way of seeking out a serious and accurate response.

For your benefit and those of other students pondering this query, I have prepared an attached workbook that cites the description of a falling film evaporator and how it works. Study this material carefully and thoroughly and note the following:

• The name of the apparatus is “FALLING film evaporator”. Therefore, by simple engineering logic, the liquid has to FALL.
• The falling liquid is EVAPORATING – just as the title of the apparatus tells us. Therefore, vapor is being generated as the liquid falls. This vapor can either rise or fall with the liquid (establish a counter-current or a co-current flow mechanism).
• If the vapor is sent counter-current to the liquid, some liquid will be held up due to resistance the vapor presents as it (the vapor) rises; this will cause liquid holdup in the vertical tubes or create a thicker liquid film.
• The basic design scope of work calls for the liquid to spend as little time getting heated as is possible in order to not thermally damage the liquid. That means ….. guess what? You want the liquid to fall fast – not slowly. Therefore, designing a counter-current evaporator is NOT what you need. What you need to do is obviously to create a co-current flow of vapor + liquid down the tubes.
• Co-current, gravity flow also allows the design to have the liquid naturally fall into the sump of the evaporator and be easily separated from the generated vapor which is drawn out and into a cyclone separator.
• The cyclone separator separates any entrained droplets of liquid subsequently carried out of the evaporator by the vapor. The characteristic of the co-current flow is to allow much easier vapor-liquid separation and produce much less mechanical entrainment of liquid out with the vapor.
• The vapor flow is induced downward by a pressure drop driving force created across the evaporator.

I hope this explanation is understood and accepted by you. This is a classical example of how actual simultaneous heat and mass flow is carried out in industry and will serve you well if you study and retain it. I also hope that you and all the students that read this thread will take an educated and respectful outlook at the manner in which you communicate with others – especially engineers. I am an old man who has been “around the block” more than several times and I know when others think they can fool me or put me off with the flimsy excuse that “I don’t speak or write English very good”. That simply won’t work with me. I know all too well that you (and most other privileged engineering students foreign to the USA or Great Britain) know exactly how to write correct and understandable English. That you don’t practice it because you think you can be “cool” or “avant guarde”, or whatever else is in current fashion is one thing; but to employ such bad communications when you are the one requesting help is adding insult to injury on those of us trying to help. Please do not try to insult our intelligence on the Forum. You will only injure your opportunity to learn something useful for your future career.

Thank you.
Click to view attachment

The reason the vapour is in dowmward flow is because usually at the bottom of falling film evaporators you will find that the vapour is condensed therefore "sucing" the vapour from the top to the bottom.

There are some explanations how it works on
www.fallfilmverdampfer.info

I hope this helped