Thermodynamics of First Crack... Continued - Page 3

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
crunchybean

Postby crunchybean » Jan 11, 2019, 4:36 pm

edpiep wrote:To you first analogy; I like my chicken juicy and tender, same with coffee I guess (fruit juicy and delicate or "tender") lol. Now granted I am not roasting 85+ point Ethiopians that can really deliver some outrageous complexities like I described but...I get your drift. I think I am going to hit the gas harder on the front end and try to drag out my Maillard longer so my FC dip isn't as extreme.

To your point about introducing fresh air/atmosphere with the correct temp to the beans; are you just saying in an obscure way to simply increase airflow before FC? That would cause the exploding water vapor in FC to be sucked out quicker and therefore decrease the dip in RoR from a "crash" to a "landing" amiright?


Yea I say these things in obscure way to try and capture that there are multiple factors to keep in mind. Like with the chicken, then "how will you cook the chicken, what tools would you use to accomplish a juicy tender chicken? Would you low temp water circulate and finish it with propane or slow smoke? Roast? There are time temp ranges with the different methods, similarly. How you can effect the heat in the roast but whatever means available to you. I don't know how you would do that, and why I was vague.

WilliamstownRoasters

Postby WilliamstownRoasters » Jan 13, 2019, 11:40 pm

Hi everyone, my name is James and I authored the paper that appeared in roast re, humidity. Just wanted to say thanks very much for your kind words and thoughtful discussion regarding the paper. I'm happy to answer any questions you might have regarding the experiment or the setup. Cheers.

crunchybean

Postby crunchybean » replying to WilliamstownRoasters » Jan 14, 2019, 12:03 pm

Hi and welcome. The paper was very interesting but are there plans in the future for more testing/mapping different beans? Did you notice during the humidity chart, where first crack to place and its duration. Can you add a little more information on how density of vapor it's relation to heat/ RH work? And lastly what have been your observations of taking in moisture loss data? Any suggestions or insights from this experiment that will effect how you would approach roasting Sumatrans beans?

WilliamstownRoasters

Postby WilliamstownRoasters » Jan 14, 2019, 10:39 pm

The system in the paper is what I run my roasts with. Every bean that I roast goes through it, so I have data on a few different origins and varietals. There is some variation largely based around age of the coffee. The peaberry behaves a bit differently too, retaining a lot of water. First crack and the duration for the curves in the paper occur at 9:18 at BT 93°C, I will say though, it seems the pattern that humidity increases before first crack occurs. My hypothesis is that initially when steam is generated it can get out of the bean without breaking the structure. However, during first crack steam is generated faster than it can be released, which builds pressure, eventually fracturing the bean. Once the structure is broken, it can be released faster, which is why there is an increase in the drying rate and large humidity spike. This could also account for the expansion of the bean throughout process, but there are also other volatile organic compounds and carbon dioxide produced as well.

My apologies, I'm not 100% clear on what you mean by density of a vapour, but I'll attempt to share what I know anyway. I think water is a very interesting substance as it's so abundant and so necessary for life, at yet, it behaves strangely. It is the exception, not the rule in a lot of respects to what we know about chemistry of other liquids. Water has this property of being a light molecule but expands by a disproportionately large amount when compared to other compounds with the same molecular weight. This is due to the hydrogen bonds in water, which are quite strong molecular bonds. Water has a high boiling point comparative to its molecular mass. This results in a molecule that absorbs lot of energy, in its liquid state, to change temperature. It takes a lot more energy to transition it into a gas, and when it does transition it greatly increases in volume. It's also abundant and therefore cheap. All these properties are great if you're trying to run something like a steam turbine, but what has it got to do with coffee?

I think that most of the energy expended is consumed transitioning water from liquid to steam. In terms of the overall energy balance of the roaster once you transition through FC the amount of energy you need to keep increasing in temperature is diminished. Sometimes I see the endo versus exothermic argument made. I don't think that the roast is ever truly exothermic, as in it produces its own energy (from chemical reactions). I think it just appears so, as it's so easy to get into that runaway state where there is so much excess energy in the machine.
With respect to stalling, the steam leaving the bean can go from a high-pressure state to a low-pressure state when it exits the bean. It may be this expansion is something like adiabatic cooling, where that expansion of water vapour in the drum is the causing the temperature decrease. Caveat though that no process is ever adiabatic. There is always some heat transfer, but it might be at least a way to get an idea of the total energy available if all the water was to suddenly go from say high pressure inside a bean, to low pressure in the drum. Also it might be good to think that if a roaster was trying to pull back on a process to prevent that runaway state, then the roaster might already be in an energy deficit. The gas might be off, there's little or no heat being supplied to the system, so instead the hot metals or hot air in the drum is in fact what's pushing the roast along at that point. If there was a sudden decrease in energy available to the coffee, say by 'endothermic flashing' (Illy), or steam rapidly expanding, this could send that balance way into the negative, resulting in a crash. If this were the case you would see more crashing with roasts with high moisture content. In Australia, it seems the African varietals are the ones with both high moisture content and also the ones that have a propensity toward crashing. In my 1kg, I don't really see it much, but my batch sizes are very small, therefore the total water in the system is lower so you may not expect to see the effect. I also installed new elements which are rated up to 4.2kW, which is a lot more energy compared to the mass of the coffee then some other roaster designs.

So how this all effects how I roast coffee is that the machine largely does the roasting by itself. I set the end temperature, charge temperature and the machine lets me know when to open the top chute, and when the roast will likely be finished. It uses that sudden humidity spike to telegraph first crack and takes control of the fan and the element. It also tells me when to drop the coffee out of the drum. I'm looking replacing these manual operations with actuators soon, so that as long as the roaster is fed coffee it will keep churning it out. The weight is also predicated using the RH probe and compared with the actual result at the end. Most times it's pretty close (within 5 grams or so), however this can drift as the stack fouls up and the readings become less accurate. So rather than having profiles for just one coffee (i.e. Sumatran (6months old), hot day), the machine looks at that change in temperature, what its prediction model is saying and adjusts itself up and down. As long as the batch size in the ball park, it will quite happily chug along by itself. Long term the idea would be to implement most of this in software so that it can be added to other roasters. I think automated roasting is very possible and is coming along much sooner than a lot of people think. This technology has been around for a long time in the hydrocarbons industry, so I think we're due. Using the mathematical model from the paper, we picked up a couple of silvers at the Australian International Coffee Awards, so I think it seems to be working. But I will end this (sorry for the essay) by saying this is just how I think it works. This is a deceptively complicated thermodynamic problem, that is complex. I'd like to think that there are definite answers, but modelling wet steam systems is really hard. I think that all models are wrong, including this one, but some are useful. So I hope this was useful in explaining your question. Also thanks for reading if you've gotten this far. I live for this stuff, which is weird and I don't even fully understand it, but I really do love it.
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crunchybean

Postby crunchybean » Jan 15, 2019, 3:52 pm

The system in the paper is what I run my roasts with. Every bean that I roast goes through it, so I have data on a few different origins and varietals.

Can you post these plots/graphs? Have you tried adding more or less heat during different segments and measuring the moisture release? Would you be willing to?


...The peaberry behaves a bit differently too, retaining a lot of water.

I think that is more a physical property of your roaster and the bean shape, maybe try decreasing/increasing the drum speed based on increasing/decreasing temp speed. Basing the alterations on contact time of drum to bean mass.

... First crack and the duration for the curves in the paper occur at 9:18 at BT 93°C,

What thermometer did you use? That temp is noticeable lower than the mean of the profiles I've seen.

..I will say though, it seems the pattern that humidity increases before first crack occurs. My hypothesis is that initially when steam is generated it can get out of the bean without breaking the structure. However, during first crack steam is generated faster than it can be released, which builds pressure, eventually fracturing the bean. Once the structure is broken, it can be released faster, which is why there is an increase in the drying rate and large humidity spike. This could also account for the expansion of the bean throughout process, but there are also other volatile organic compounds and carbon dioxide produced as well.

That is generally what I think since the bean structure goes through a mailable plastic states and glass state which contribute to the overall phenomena. And to me is the reason the flick is at the end.


My apologies, I'm not 100% clear on what you mean by density of a vapour.

I felt like, since you are using the density of water vapor at a given temperature as the control. (I may have missed this) But are you cross checking the numbers by calculating the the total volume of air compared to the water saturation? Taking that total amount of water and comparing it to the beans mass before/after and seeing if that aligns with the weight of pure water and how much it varies from the weight of pure water.

WilliamstownRoasters

Postby WilliamstownRoasters » Jan 17, 2019, 1:23 am

Try this https://1drv.ms/x/s!AoSsr0gABTPUgsFkHCIit3uV7YcBlQ for a few curves, these are the same ones from reddit. I think the heading is wrong where exhaust and bean temperature is swapped. There's about 700 or so of these text files that are basically CSV files. there's about 2000 or so pdfs

https://1drv.ms/f/s!AoSsr0gABTPUgsIevYTc_vMhF32r5g <- there's a link to a folder with some of the PDFs. Can't say they'd all be useful There's some in there that are just wrong due to fouling in the stack, or issues with the controller. Most should be fine though. It might be a fun thing (depending on your definition of fun) to whack them all into a big database or something and have a look based on varietal etc, but not something I have time for right at the moment.

The roaster constantly varies its heat up and down through the roast. This doesn't seem to affect the drying rate, least not within the period of time the coffee roasts. It might be a good avenue for some experimentation though. Through observation longer roasts with smaller peak ROR's tend to lower drying rates than really hot, quick roasts. But dropping the element from say 100% to 20% right at the end of the roast doesn't seem to affect it too much. Though that is definitely worth playing around with.

The roaster uses k-type, ungrounded, 3mm thermocoulpes. Readings can be vary based on where the probes are in the machine. So it's a little hard to compare temperature values directly. Mostly first cracks seems to be in this area +/- 3%. The variance could be related to different mechanical design and placement?

The probe reads relative humidity, so the partial pressure of water in the gas mix compared with the equilibrium vapor pressure. If you use some existing data to know that saturation pressure experimentally, then you can get the total concentration of water. So you essentially calculate the saturation pressure, and use relative humidity to give you the vapour pressure of water in the air. From there you can get absolute humidity, which is the mass of the water in a given volume of air. I think that's functionally the same as density. If you work out the volumetric flowrate of the air, then at any instant you can know how much water is in the stack. You then approximate the total by taking the integral of that number over the length of the roast. With the control experiment, it was just there to check the method was working. It's not used in the calculation, though it could be.

I mean theoretically, just evaporating pure water should give the maximum drying rate. Any difference in this number should be due to the heat transfer characteristics of the coffee. So that might be something interesting to check. A friend of mine was talking about hitting some coffee with a thermal camera to see over time how that heat transfer changes, but this might be even easier. I think I've got an idea for a new experiment...