[dennis13]

Hello,

What kind of apparatus do I need to produce a water/kaliumhydroxide/lignin solution to near supercritical conditions?
Should I heat up and increase pressure within one apparatus or should I do both processes seperately (demanded conditions: 623 K, 19 bars), with the aim to roughly estimate costs for conducting hydrothermal liquedefaction in order to win catechol on an industrial scale? I am writing a homework paper for my bachelor degree.

If I should heat up the solution seperately, would there be any steam or fluid able to heat up the solution in a shell/tube heat exchanger or would I need to heat it up by some kind of combustion apparatus / furnace? Since I need to heat the solution up to 623 K and thermal oil in a heat exchanger works only up to around 673 K I doubt whether its efficient to use. If I would use steam in a heat exchanger it might has the disadvantage that steam generally has a low heat capacity. I have read that there might be safety issues by direct heating organic solutions, because they are easy flammable. Hence I am also questioning the use of a furnace.

Best,
Dennis

[Enthalpy]

Hi Dennis,

a few general ideas, because I haven't fully understood your query.

Did you check if water (with potassium hydroxide) is liquid (as you want a "solution", or possibly a suspension) under these conditions? 623K=350°C is near the critical point of 374°C and 218bar for pure water, so 19bar look very little. Or is it 19MPa rather?

For a reaction in a liquid medium, you don't want to heat first and put pressure later, since 350°C would evaporate the water.

Near its critical point, liquid water has expanded by a factor of four. The apparatus must cope with this.

Oil for a heat exchanger too must be heated first and this costs, unless you have waste heat elsewhere at the production plant.

To reduce the heating costs, recycle the heat from the reactor's output to pre-heat the reactants, and add costly heat only afterwards. This needs an exchanger, which isn't trivial to keep clear if you use slurries.

You might seek a liquid accepting higher temperatures than oil. And I would question the fire hazard of the oil too, not only the reactants and products. Catechol is volatile: flash point +127°C, autoignition 510°C.

"Steam has a low heat capacity": you should put figures on that. Condensing it over 350°C brings supposedly little, and I see too little temperature margin for liquid water, but vapour does carry heat, so compare with your needs.

Maybe (but maybe not!) you could heat the water (plus hydroxide optionally) separately, far from the flammable reactants and products, and introduce it in the reactor to heat the rest. Potentially safer, but proper mixing is more difficult under the reactor's conditions, and introducing a solid too.

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Could someone explain me the English Wiki's data about catechol?
https://en.wikipedia.org/wiki/Catechol
Melts at +105°C, boils at +246°C (sublimes).

[dennis13]

Thank you very much for your extensive and helpful response. I indeed meant 19MPA, 190 bar.

Would you think its meaningful to preheat the feedstream by cooling off the products and use a furnace driven by combustion gas to heat the feedstream up to 350 C? Dont know whether its possible to coil the pipes with the feedstream under 190 bars around the combustion chamber of a furnace.