Posted 10 March 2004 - 11:29 AM
Noora:
Thanks for responding with the additional explanation. Now the picture takes on more clarity and understanding.
I designed an analogous system in 1971 that was very similar to what you are studying. Perhaps it can help to guide you.
My problem was to vaporize an equilibrium quantity of Furfural liquid into a stream of hot Hydrogen and subsequently feed the resultant vapor mixture to a fixed bed reactor where I produced Furfuryl Alcohol. Prior to this, the operation was being done in a small, packed tower. What I designed and built was similar, but bigger, and is still operating as 2 production trains in Geel, Belgium. I don't know if it has been up-graded or improved, but it worked then and continued to do so for many years. I understand it is still functioning as a showcase plant, 33 years later. It's a source of pride and nostalgia for me.
The problems I found were:
1) The operation is basically one of simultaneous heat and mass transfer; it is very similar to what happens in a cooling water tower; however, in this case, there are little or no mass transfer rates available and the operation had to be piloted - in other words, you need empirical and experienced field data.
2) Although the operation is one of simplicity and involves a minimum of hardware, the results can be very complex and difficult to control and operate. Let me explain further, because this is important: you must use or employ some expanded surface media - such as packing (or trays) - in order to facilitate the intimate contact between the two phases. Countercurrent operation is more efficient, but you may have to use parallel current if you have a heat sensitivity issue with one or both of your phases. The time required for contact is controlled by the mass transfer coeficient and not the heat transfer. Therefore, the diameter and height of the "vaporizing" tower is what you have to identify.
3) The operation becomes complex because of the nature of the fluids: if one of the fluids is heat sensitive and can polymerize (as Furfural did, in my case) you have a serious problem. Additionally, you might form some side reactions. What this translates to is that you have a critical decision on the type of tower internals that you should use. Any impurities in the liquid will precipitate or clog and plug your packing or trays. Maintenance will become a nightmare and operation will be very difficult to control. You must prepare for this or design around it - otherwise, your design will be un-economical and fail.
4) What is going to happen in your proposal is that you will soon start to concentrate impurities in your re-circulated liquid and these will reach a point where they will precipitate and plug up your system - especially any atomizing nozzles that you employ. The worst event is when the packing plugs because you have to stop and evacuate all the packing and clean it. Trays are similar and probably harder to maintain. But both will plug inevitably if allowed to, especially if a process design is not incorporated to keep the liquid fluid clean and homogeneous.
Basically what you are doing is "humidifying" your Hydrogen stream with the liquid - so the operation is one of simultaneous heat & mass transfer. Your text book on this and on the design of cooling water towers will cover the subject. You should design the system under this scope and not have to resort to classical heat exchanger technology - except for the pre-heating of the liquid and Hydrogen.
If you have ever operated or seen the operation of a simple water humidifier, you can appreciate what is happening. A person can never imagine the amount and quality of impurities that abound in water until they see the results of a clogged-up water humidifier that has been fed water with dissolved solids. The same effect will ultimately take place with an industrial humidifier - or "vaporizer" as you call it.
That's about all I can comment on, since you obviously haven't given us all the detailed basic data. But I hope this helps.
Art Montemayor
Spring, TX