Furfural heat exchanger tubes 1

[Soldador]

Hi guys,

I have a problem with a dry furfural condenser used in the lube plant to separate certain kind of lubricant. Inside the tubes, a cold mix of furfural (solvent) + hydrocarbon (containing the lubricant to be separated) circulates, while hot furfural circulates in the other side.
The main problem we have is the formation of an adherent product on inside wall of the tubes (carbon like product), which eventually can obstruct or close the tube completely.
We want to change the current material of the tube (Alclad 3003 aluminum alloy). We think that AISI 316L stainless steel is a good candidate. However, we are afraid that stainless steel can make the problem of the sticky product even worse. I used a "kitchen" analogy to explain this: comparing copper and carbon steel pans, it is easier to cook using the copper pan, because the food does not stick on it. This phenomenon is due to the higher conductivity of copper.
In the same way, I am afraid that the fluid can stick on the stainless steel much easier due to the lower conductivity
Thanks for your time

 

[25362]

Is this thread connected with thread124-107180 ?

Furfural polymerizes into a coke-like sticky material in the presence of air, acids, or suitable temperatures. Furfural already contains two oxygen atoms in its molecule, a fact that predisposes it to chemical reactivity.

To minimize this effect keep furfural acidity low. Keep also control of the "napthenic" acidity of the feed. Deaerate feedstocks. Check for the presence of iron chloride in the feed. Control the quality of the inert gas.

As a, not always, practical approach try to increase linear velocities as much as possible; one may be obliged to re-design the exchangers and parts of the solvent recovery system.

As for the metals, because condensing furfural has a much better heat transfer coefficient than the colder non-boiling (?) solvent mix the tube wall (metal) temperature is bound to be nearer that of the warmer side whether you use CS or SS 316L or SS 317L.

 

[Soldador]

Yes, this theread is releted with the other one
Thank you very much for your quick and helpful answer

 

[25362]

Soldador,

Please note:

1. If the plugged up exchanger is on the raffinate-mix side, there is good chance to find polymers in the extractor itself. What type of extractor is in use ?

2. The degradation/polymerizing reaction is exothermic and self-accelerating, as a chain reaction would be, by heating itself up.

3. The oxidation stability of the base lube oil may be compromised by traces of furfural, even after a hydrofinishing step.

 

[Soldador]

I do not know the process in details. I also do not know if I follow what you say. sorry but this subject is not one of my strong point. What I can tell you is that furfural is used as a solvent in the manufacturing of some lubricants. After all the typical destilation processes, the remant product contains some lubricants that can be extracted from it. This equipment has the objective of saving energy in the following fashion: fluid from a circuit is hot (furfural), then it is used to heat up another fluid (furfural+ fluid mixture that contains the lubrican we want to extract)
Operating conditions are the following (aprox)
in out
P[psi] 33 33
T [ºF] 320 410
The equipment itself is a regular heat exchanger

Hope that this information answers your question

 

[25362]

Whatever the process is, to heat the furfural mix up to 410^oF would necessarily mean the tube wall would be warmer.

Is the reported pressure gage or absolute ? Depending on that and the actual molar % of solvent, the mix could start vaporizing. A two-phase flow condition would complicate matters even more.

If one or both of the components of the mix (oil or furfural) contain dissolved air or acidity of some kind, degradation and sticky polymer formation is to be expected.

The degradation and polymerizing reactions form more acidity, and by being exothermic they accelerate, sometimes at very high rates, as chain reactions would.

 

[25362]

Soldador, allow me a short review on the reported in/out data.

1. If the pressure readings refer to the tube side and there is flow, they should differ by about 5-10 psi o/a of friction drop. Could it be that the gages are plugged up and the readings aren't correct ? Please comment.

2. The shell side which has, as I understand, a condensing dry furfural vapor slip stream, would have to be at a pressure of the order of 55 psig to allow for a condensing temperature of, say, 430^oF, with about 20^oF above the reported mix outlet temperature. Kindly confirm.

3. Where is the heated oil-furfural mix discharged into ? Is it by chance re-heated prior to being fed into a (vacuum) flash vessel to recover the solvent ? Please let me know.

After receiving your comments I'd be able to better understand your process.

 

[Soldador]

sorry 25362 for being late, I was out of town.
Regarding to the gauge, it is possible, I do not have the chance to check that.

Regarding to the condensing temperature, I double check that information and it is correct.

you are right when you refer to where the mixture is discharged into.

I´d wonder if I can get a source of information (book, web, etc) to be more informed about this, so I can interact with you better.

I really thank you

Gustavo.-