3 replies to this topic

[NiallS]

Hello there,

 

I'm currently designing a Continuously operated Slow Pyrolysis Reactor for one of my classes and have reached a point where all the kinetics are modelled.

 

A key factor of Pyrolysis is the heating rate, with a a lower heating rate yielding more Biochar than a higher heating rate. (This is what my kinetics show and this agrees with literature.)

 

My kinetics were modelled using a constant heating rate in MATLAB, I then used this model to evaluate how long it would take to convert all of the biomass, using the heating rate and residence time, I was able to calculate the final temperature within the pyrolysis reactor.

 

My confusion comes when trying to select the correct heating rate to maximise cost effectiveness.

 

Below is some data which may be useful to understand my problem;

 

As an example, at a heating rate of 0.1 degrees celcius/minute, my time to reach full conversion of the biomass is 1630 minutes, the final temperature is 302 degrees celsius. At a heating rate of 1 degrees celcius/minute, my time to reach full conversion is 222 minutes and the final temperature is 361 degrees celcius. Lower heating rates produce more char and as char is my intended product I want to maximise its production.

 

If I use the general energy balance equation,

Q=mc_p(T₂-T₁) with m being 1, c_p also being 1 (assuming constant over temperature range), T₂ being the final temperature and T₁ being the starting temperature (139 degrees celcius)

 

I get;

Q=163kJ for the first scenario and Q=222kJ for the second scenario, converting these to power, by dividing by the total residence time in the reactor I get 0.0016kW for the first scenario and 0.016kW for the second scenario.

 

Surely there will be a higher energy/power requirement to keep biomass in the reactor for longer, even though it reaches a lower final temperature? Am I overlooking something? I feel like I've been staring at this for an eternity.

 

Any help is greatly appreciated,

Niall.

Edited by NiallS, 08 March 2019 - 01:38 PM.

[Pilesar]

Are you taking into account any heat and material losses? Did your draw your material balance envelope around the process and include every inlet and outlet, including any off gas? You are not just heating a mass -- you have a reaction to consider. Did you account for the heat of reaction? Where did you get your assumption that heat capacity is constant 1? Can you verify that assumption? Would that be reasonable for the feeds and all the reaction products?

Edited by Pilesar, 08 March 2019 - 02:18 PM.

[NiallS]

Thanks for your response Pilesar,

 

I was trying to simplify my initial thoughts before I went through a detailed energy balance, Here is a link to an image of the spread sheet I am using for my analysis;  http://prntscr.com/mv7dqt

 

As you can see the effect of heating temperature has a fairly low effect on the composition of the products being formed, however as char produces the most revenue it would technically make the most sense to use an infinitely small heating rate to maximise char production, however this would require an infinitely sized reactor so this is infeasible.

 

Moving on to the question regarding heats of reaction, to quickly analyse the effect of heating rate on energy use, I'm ignoring them when comparing the energy requirement as the product distribution is similar, I'm assuming the heats of reaction will be similar. 

 

Finally regarding the assumption that heat capacity is 1, that's simply a random number I selected to illustrate my point.

 

 

I believe the crux of my question can be simplified by ignoring the heats of reaction for now to,

'What are the implications on energy consumption when comparing heating of a material at a slower constant heating rate, compared to heating it more quickly to a higher temperature?'

 

I've asked class mates and they seem to think that heating something more slowly to a lower temperature is more efficient but to me this doesn't make sense to me, why not use the slowest heating rate possible that provides a feasible reactor design?

[Pilesar]

Infinitely sized reactors are very expensive and probably not cost effective. There is much about the problem and your work on it that is still hidden from me. Were you given the operating envelope of your process? An unconstrained optimization tells you something, but you may get a different solution when you add proper constraints. There are limits on how fast you can heat the reactor and how fast you can load and unload. Using arbitrary numbers in engineering equations can give nonsense results. If you use realistic values, you may be able to locate which areas don't pass a 'sanity check'. Matlab can be misleading in that there is a sense of accomplishment when all the equations solve, but that does not necessarily mean the answer is useful. Does your reactor have any heat loss to the environment? Is there any cost of capital? Is your mass in the reactor constant or is the gas product leaving during the run? Heat capacity for the gas is surely not really the same as for the char and tar. Because the char and tar is different for each scenario, the reaction heat will be different also. Are you considering a cost of energy? This looks like an interesting problem to work on. I suspect there are multiple errors in your approach, but this problem is for your learning and you should take advantage of the opportunity. I don't know enough about the problem to point you any closer.