2 replies to this topic

[Limilicious]

I'm trying to figure out the maintenance side of falling film evaporators; like how long can a falling film evaporator operate before it needs to be taken down and cleaned due to fouling.
I know it'll depend on the liquid that is being evaporated and the content inside. The application is for thin stillage (from corn fermentation to produce ethanol).
From the feasibility study I did on an ethanol plant, the falling film evaporators ended up being 12 meters high and several meters wide (diameter).

If possible, the amount of time required to clean the tubes would also help. The tubes are about 35 mm diameter. These numbers (12 m and 35 mm) are taken off a manufacturer site.

I found a source that said cleaning tubes takes about 5 to 10 minutes each (18 meter long tubes). I'm not sure whether that is applicable here.

I did a bit of research but most the time I can only find "vague" and "relative" answers like "low maintenance time", "low downtime", "easily maintained", "resistance to fouling". Well what does resistance to fouling mean with actual numbers? Fouling every week? or like every month?

Any help is appreciated.

[Art Montemayor]

Limilicious:

One thing that is most important to take in with total humility and understanding while you are a student is that after all the studying, hardwork, experience, and brilliant problem solving, we engineers are just plain humans and subject to being successful to the degree that we have accurate basic data and specific experience and input information. We cannot produce miracles and predict the future in those areas where we lack experience.

Of course you are coming up with "vague" and "relative" answers as to how long can a falling film evaporator operate before it needs to be taken down and cleaned due to fouling. You should already know that “corn” is not simply just “corn”. My Mexican ancestors developed and perfected the present day kernal of American corn. And they didn’t just stop with ONE version of it. There are probably a couple of thousand varieties of corn – differing in kernal size, shape, color, taste, composition, hardness, fermentaion yields, etc., etc.. The exact same phenomena exists with the original Peruvian Potato. You simply cannot generalize and expect that all species of corn varieties will behave and yield the same results when subjected to being processed. There are perfectly good and logical reasons as to why there are different type and varieties of corn. The point and fact here is that you must expect different answers for what your generalized query is seeking. There is no “standard”, “conventional”, or “rule” that will tell you a magical, universal answer as to how much fouling or processing problems you have with all varieties of corn.

You may argue, “But then, how are we expected to design a process if we don’t have the absolute, specified conditions as to how the raw material will behave?” The engineering answer is that if we don’t have the money, time, effort, opportunity, or previous information to obtain basic design data, then we have to add what is defined as a “contingency”. A contingency is something we allow for because we know – or strongly suspect – that we will need for the design to work appropriately. We use it because we don’t have the pre-determined information immediately at hand, don’t have the time, opportunity, money, or available resources to obtain the information prior to finishing the total design. Therefore we add a contingency to the design. In your particular case, we enquire other sources and make an engineering “sound judgment” and add a contingent amount of evaporator area. We also add a contingent amount of time required to accomplish any expected or anticipated maintenance. That is all we can do – we can’t produce a miracle. And even if we could, we can’t be paid enough money for producing miracles.

My serious and professional advice on this specific subject is: Don’t spend time and effort trying to force specific data on a generalized subject. When you are dealing with such unknown basic data (as often occurs when dealing with agricultural products or raw materials) you have to take risks and make calculated guesses and add contingencies. You cannot be expected to generate a “100% correct answer”. The best answer is the one that works. And the best answer often comes from a lot of prior experience and empirical know-how in the field from veteran engineers and designers. You don’t employ engineers who are “fast learners” to design such a process. You apply learned and experienced engineers who have “have been there, and done that”. And that usually results in an acceptable and productive result. This is called applying process “know-how” and it comes at a higher-than-normal engineering cost.

The reason your research is only turning up vague answers or comments is simply because the real, hard-earned answers will cost you money to obtain. No one who has obtained such know-how is going to give it up without charging for it. That is just common horse sense.

[kkala]

I quite agree to the previous post by Art Montemayor; specific knowledge is generally costy and not easy to find.
As said, Lumilicious need not worry much about downtime, since in student exercises a deviation from real world can be tolerable, it is systematic thinking, consequence and sound physical principles that mainly counts. If the exercise concerns a feasibility study and preliminary design (I imagine so), I would merely assume an operational availability (e.g. 90%) out of the impression got from literature and state it in the report. If you want more information on the subject, you may find it in the WWW, through interpretation.
For instance, http://www.barr-rosi...evaporation.asp says that a full cleaning cycle (excluding mechanical cleaning) of stillage Calandria evaporator takes up to 20 h; so you can assume a day (24 hr).
Frequency of shutdown for such cleaning depends on the design, nature of stillage and its solid content. Experience could help, if somebody happens to operate such evaporators. But you can have another assumption concerning frequency by iterpreting Web sources, if this info is necessary. A general maintenance shut down every(say) year may also be necessary.
In the past I worked (1975-81) in a ferlilizer plant, having a falling film evaporator to concentrate NH4NO3 (never dealt with it). I do not remember any serious scale problem, and there was no spare evaporator, so I could imagine a frequency of once per month or less (and not for 24 h). Such stops for cleaning may be according to a determined program.

Edited by kkala, 27 November 2010 - 07:03 AM.