Specific and Latent Heat - Water/Alcohol Mix

I'm guessing you're not a moonshiner. Water/ethanol mixtures will vaporize in specific azeotropes.

And why in God's name would you heat wine to the boiling point? You'll destroy the taste.

TVP45 as guest

Maybe my word choice was wrong - this is a potable alcohol distillation (batch) process and we are experiencing cooling problems. To produce the brandy, the wine needs to be heated to evaporate the alcohol. Obviously this then needs to be condensed and cooled. This is not happening correctly and losses are experience due to high condenser temperatures.

Are you running your condenser unit through a bath of coolant? This is usually done to stop vapor loss. Also, it's why distillation may need to be repeated several times.

You mean it's still vapor when it comes out of the worm? You need a longer worm and/or a bigger flakestand. But, which azeotrope is doing this?

It is condensed liquid coming out of the condenser, but is not pure alcohol - it is a water / alcohol mixture. Some water is evaporated with the alcohol. Highest concentration is at the start of the distillation and the weakest is at the end of distillation.

Process briefly: An indirect steam heated distillation pot contains wine at 12% alcohol. The evaporated vapour is used to preheat the next batch and then continues to the condenser. It enters as a vapour and exists as a sub-cooled liquid. The condenser is a water bath with cooling water entering at the bottom and overflows out of the condenser. Why? - It is an old system!

Ignoring for a moment the preheater and assuming total load is dissipated in the condenser. The heat load is a two part - condensation and cooling. To perform this the, latent heat of vaporisation/condensation and specific heat are required. What are these values?

Any suggestions?

Well, of course it's not pure alcohol. It's not going to be unless you go to a fractional distillation process. Near the beginning you see the high wines, then you move toward the low wines.

What azeotrope are you trying to calculate?

Azeotrope? - I am on unfamiliar terrain. Ethanol/water distillate as product of a first distillation of wine - does that answer the question?

My approach to the problem was that if it was water to be condensed, the latent heat for water (2257kJ/kg )would be used to calculate the heat to be absorbed by the cooling water. If it was pure alcohol, latent heat for ethanol (855kJ/kg) would be used. Since it is a mixture the latent heat for the mixture is some combination (?) of water & ethanol's latent heats and this changes from the high wine to the low wine. For this reason I inquired about the latent heat for the mixture.

Maybe I should ask: What is the cooling required to condense and subcool the vapour to ambient temperatures (20degC) produced by first distillation of 12%(alc) wine?

OK, I got it. For any binary azeotrope, use the percentages. Say you have 40% water and 60% ethanol by weight, multiply each by the percentage. Add them and you have the latent heat for that azeotrope. It's better to work in volume, but you can use kg if you're used to that.

Cheers.

Thanks very much!

The problem is most probably because of the cocurrent flow in the condenser. Can you try feeding in the vapour at the top?

For all practical purposes treating the whole mix as just water (for heat calculation) will be enough.

Bioramani

Flow is counter current. The condenser is a bath type with the spiral "worm" (tube) inside. Vapour is fed from the top and runs down the spiral tube and exists at the bottom. The cooling water is fed from the bottom and overflows at the top.

Ah, and I forgot. Remember to account for radiation loss. That's often the problem in this kind of calculations (at least amongst us hillbillies who think Stefan-Boltzmann was a revenooer ).

Once again an assumption - the ambient temperature is so high that the radiant heat loss is for the moment regarded as negligible. (But as we all know - assumption is the mother of all problems) Ambient temperature during distillation season is 35-40degC and even higher inside the upper section of the building where the condensers are.

The numbers I try to calculate is to check the cooling system (which is under-performing) info obtained from the scada system. Average temperature rise is 15degC for the cooling water. Flow rates were determined during flow tests. I try to compare the haet load obtained from the cooling water system info with the cooling required for the vapour (as stated in the first posts). So far the numbers to not add up and I need to find out why.

My apologies. I stand corrected.

We then need some design info on tube dia, length, water quantitty, inlet overflow temperatures as well as product volume for making heat flow calculations. In case the heat transfer area is insufficient is there enough space in the water bath to put a few more worms in the can as it were, in parallel?

It is presumed that there is no active agitation of water in the bath. The situation is similar to steam heating coils in unstirred tanks for which calculations are available in various Chemical Enginnering books including Perry's Hand Book of Chemical Engineering.

The overall heat transfer coefficient in such systems is more controlled by the film coefficient on the water side.

Given my previous post, the situation is more one of a comparison than actual design and as such I did not venture into the heat transfer calcs as suggested. It will check Perry - missed those examples -thanks.

This is a very tricky question.

Volume ratios with these liquids are problematic, you must use mass. What I mean is that 30 ML of water mixed with 60 ML of ethanol does not get 90 ML of solution. it is less because the water "fits between the alcohol" and gives off a little heat.

Try mixing 180 proof white lightning with water measure the temperature at the start. if you mix 50 mils and 50 mils you end up with 85 MLS(?) and a warmer solution. The weights will add up.

In your case if you start with 30 ML of water and add alcohol until you get 90 MLS of solution, you added more than 60 ml of alcohol.

The latent, and specific heats for the mixture are very close to the weighted average of the latent ( or specific heats) of mixture. (The difference to exact is the small heat of solution that caused the warming effect mentioned above). Look them up in Perry's, CRC, wiki or any other reference.

Unfortunately, today's students may just plug in the variables to Chem-cad and get an answer without understanding where it comes from. Hope this helps.

Paddler

A water-and-ethanol mixture will never produce a distillate that is stronger in ethanol than a [_forgotten] percentage. In order to effect complete separation, a third component is required, benzene being an example, as is fractional distillation as mentioned above. Benzene, however, is toxic to biological systems, so the distillate from a benzene mixture needs to be applied to some other use than for consumption.

Recommended reading: Perry, "The Chemical Engineer's handbook", any edition. Look up the chapters on distillation. They are a good cure for insomnia, like ethanol.

Thanks guys!

The product we are producing is good old brandy. It is distilled once up to 30% strength (average for the batch) and then distilled again some time (days) after the first distillation. For this second distillation only the "heart" is used (the middle section of the batch at about 60%). Benzene is therefore not in the picture.

The required cooling (calculated with the weighted Cp & latent heat) is less than the actual heat transfer calculated by using cooling water inlet and outlet temperature and the water flow rate. This allows (I guess) for some safety factor and possible radiant heat transfer to the condensers during the high temperature summer months.

I appreciate your comments - this was my first participation in the forum and the response was great.

I have a couple of bottles of 200 proof reagent grade ethanol... Weird stuff!!! Oily, very nasty, completely undrinkable - it will close your throat on contact, and it'll set aluminum powder on fire!!!