Re: GAS-L: Re: law of mass action

[Reedtb2@cs.com]

Dear All:

More to the point of the effect of pressure on pyrolysis:

Some (but not all) of the reactions during pyrolysis involve vaporization of the various monomers, fragments and ologomers of the cellulosic/ligninic components.  The rate of vaporization is VERY much dependent on pressure.  When you exceed the pressure of the atmosphere it proceeds cataclysmicliy and is only limited by the rate of heat transfer - which may be much reduced by vaporization (as in the case of a pot of water heated from above with a torch) - or may not (when the pot is heated from below).  

So I believe Mike Antal's new high yield charcoal process depends primarily on reducing the rate of volatilization of the components at higher pressure so that they can undergo charring reactions inside the matrix rather than being carried off with little chance of further reaction.  It is also common experience that the yield of charcoal can be much bigger for a large chunk of wood than a small, since the vapors can spend more time in the matrix....

Yours,       TOM REED

In a message dated 1/26/02 4:07:28 AM Mountain Standard Time, kchishol@fox.nstn.ca writes:


> > OK.... given that pressure has little effect on "reversing a reversible reaction" at the 400 C range, is it not likely that higher pressures could act as the equivalent to a "negative activation energy" to prevent the reaction from occurring in the first place?
> >  
> > As a "Laymen Example".... a little spray of water will extinguish the match, prventing the firecracker from "reacting." However, if the firecracker has "reacted", then no amount of water spray will reverse the reaction.. 
> >  
> > It would thus seem obvious that pressure applied before an irreversable gassifying reaction occurred would have a reasonable expectation of yielding good results, by potentially preventing  the undesired reaction. 
> >  
> > Does that make sense?
> >  
> > Kindest regards,
> >  
> > Kevin Chisholm
> >  
> >
> >
> > Dear Paul: I agree with Tom's comments.  Most of my work has emphasized somewhat lower temperatures, where thermochemical equilibrium is not often achieved.  It is usually instructive to compare observed results with thermochemical equilibrium predictions.  As one approaches equilibrium, dramatic changes in the product slate become less likely.  But in my experience (at lower temperatures without catalysts) the equilibrium calculation is only a rough guide, that can offer insights in the absence of experiments (i.e. its cheaper to run STANJAN than to do an experiment).  For example, detailed thermochemical equilibrium calculations predict a negligible influence of pressure (above 1 atm) on charcoal yields from cellulose at 400 C.  As my work has shown, this prediction could be quite misleading.  Best regards, Michael. 
> >
> > >