6 replies to this topic

[Arbojl]

Does anyone know a good resource on the theory and operation of wiped film evaporators? I am having some issues with heat transfer on our unit in the plant and am trying to get to the heart of the theory.

I have looked in Perry's and did a search on Dogpile, but I haven't gotten the kind of information that I am looking for. Our basic issue is that the evaporator seems to perform better as we 'thin' the product out with water. I understand the change in viscosity will alter the heat transfer; however, I would like to dig a little deeper into the theory of operation.

Thanks!

[Chris Haslego]

Wow...going to be difficult to find actual correlations on that one. Usually, the problem is that people have the theory and want to know more about the practice. Since you have a unit is operation, why not plot the viscosity of the feed against the overall heat transfer coefficient?

You're correct in that the reduced viscosity is certainly helping the heat transfer. Other factors at work may include the fact that (depending on the vapor rate), you may be crossing into a shear controlled flow regime which is adding to your heat transfer. The film may be acting in the transitional or turbulent flow regime at very low Reynolds numbers (a common scenario for thin films). The vapor shear aids this regime by inducing a "wavy" characteristic to the film (if that's what's happening in your unit).

Since it's nearly impossible to really say whether the reduced viscosity or a change in flow regime is primarily responsible for your enhanced heat transfer, I'll refer you back to my first paragraph. But I will say that a reduction in viscosity (even a small one) could have a great enhancement on an evaporator that is largely limited on the process side.

[Alex189]

Hello, I've just installed a pfaudler wiped film evaporator for purifying fatty acids (mainly beef tallow) but have run into some problems.

Firstable, it's not quite clear to me how the evaporator works. In the fatty acids you have caproic, estearic, linoleic, etc, each with different boiling points. The fatty acids with the low boiling points are the ones you want to get rid of since they are the main constituents of odor and dark colors. From what I read on evaporation theory, that the wiped film evaporator makes a thin film so as to create a better transfer of heat and evaporate the material a lot easier. The evaporated acids go through the entrainment separators, are condensed and finally pushed through the destilate output. The ones that did not boil are sent to the bottoms outlet.

My question is: If, I want to get rid of the low boiling fatty acids to get better color and odor, shouldn't I set the temperature just enough to boil those "low boiling" fatty acids and let the others flow through the bottoms to be my purifyed product?

Obviously I'm not working that way. I set up the temperature to boil everything and what I seem to have difficulty in controlling is the amount of condensed fatty acids to the bottoms. The ratio right now is 1/1 !!! That's terrible! I'm recirculating the bottoms ofcourse but I fear this kind of results are not what the evaporator is meant to produce. I have tried to reduce the feed flow rate and it gives me a better ratio of destilates to bottoms but the production rate is way low.

Here are some paramaters I'm working with:
Vacuum = 71mmHG (as shown in gauge from 1 to 76)
Thin Film Temp = around 270 C.
All the bottoms are recirculated to the pfaudler evaporator. Don't know if recycling this product over and over will eventually degradate to a point were the pfaudler can't purify it.

Any help on this will be deeply appreciated.

Alejandro Elizondo
Monterrey, Mexico
Compania Mantequera Monterrey S.A de C.V
www.agrocorp.org

[pawan]

Arbojl first

The theroy for heat transfer is always same independent of equipments only correlations change. Probably if U can share ur real problem we can give out our inputs to it.

Alex

I am confused bcoz first U said distillate & bottom ratio is 1:1 & then U said all bottoms are recycled.
Can u make it more clear by a sketch or following info.

Feed to WFE with fraction of low boilings
Distillate U r getting & its temp
Bottoms U r getting & its temp

Desired condition

Though it is surprising that U r operating WFE at 270°C normally it is used for degradable heat sensitive products but if ur product can withstand this kind of temp U could have used distillation also with falling film evaporator ( I dont know I may be wrong) but at lower vacuum.

[Alex189]

Hello Pawan, thanks for your reply.

What I meant by recycling is that when the fatty acids are fed from the primary holding tank to the WFE they are basically separated into destilates (the purified fatty acids) and the bottoms (fatty acids that did not evaporate). From what I understand, if the product was not evaporated it may be because I'm making a film that's to thick and some material will not get the sufficient transfer of heat to actually evaporate. Being that the case, the bottoms are recirculated to the primary tank where the WFE gets its product feed.

What I meant by ratio is that the destilates tanks are filling up at the same rate as the bottoms tanks when I believe the pfaudler should give a ratio of about 10:1.

Regarding the temperature of the bottoms, you can actually notice it is a lot higher than the T in the destilates but that's mainly because there is no water cooling system for the bottoms.

[pawan]

The fatty acids with the low boiling points are the ones you want to get rid of since they are the main constituents of odor and dark colors

My question is: If, I want to get rid of the low boiling fatty acids to get better color and odor, shouldn't I set the temperature just enough to boil those "low boiling" fatty acids and let the others flow through the bottoms to be my purifyed product?

fatty acids are fed from the primary holding tank to the WFE they are basically separated into destilates (the purified fatty acids) and the bottoms (fatty acids that did not evaporate)

In view of above I am still not clear if U want top as product or bottom as product bcoz both the posts are opposite. Anyway One by one my reply is as below.

I believe the pfaudler should give a ratio of about 10:1.

1. Design does not work on individual believes. First confirm the design data.

and the bottoms (fatty acids that did not evaporate)

2. Secondly know ur feed concentration then only u can say that bottoms did not evaporate. What are the parameters P & T in design for this WFE system. How much fraction is low boiling & how much is high boiling.

Being that the case, the bottoms are recirculated to the primary tank where the WFE gets its product feed.

3. This can help only if it still has low boiling components. It is possible that bcoz of recycling U r getting lower ratio of top to bottom. Bcoz if U stop recycling ur feed will go down by 50% & probably U may get better separation bcoz now load is reduced. So try to stop recycling for few hours & see the result. It should be run till all the bottoms from feed tank have been depleted.

The reason for above points is that your doubt of higher film thickness & poor heat transfer is possible in the following cases with these prime reasons.

1. higher viscosity
2. higher feed rate
3. lower scrapper rpm
4. higher pressure.
5. change in feed composition

So try to match these parameters with design condition & then observe. Bcoz currently from ur description only item 2 is applicable that is higher feed rate due to ur recycling


Yes bottom temperature is the final temperature where separation is taking place (This is important not the top vapors temperature which are generally cooled inside) & probably you dont need cooling water for bottoms in WFE. It should be outside the WFE for cooling of bottoms either as a product or as a waste.