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Boling water from a wood slurry using the ideal gas law

In our course lab, our experiment required us to take wood fibers and mix them into a slurry in which that slurry was 65% water content. We then formed the slurry into a brick and placed it in a small electric lab press where variable hydraulic pressure and heat energy was applied to this wet wood brick.

Using a thermocouple and pressure probe, we measured the internal temperature and gas pressure in the core of this wooden brick.

I have attached the data file and the graph which shows the application of hydraulic energy and the resultant core temp and internal gas temp as a function of time in this little lab press.

Assuming the water vapor is behaving like an ideal gas, we are asked to explain how PV=nRT can be used to derive the unknown gas volume with the information at hand. In this case, we neither know V or n - how can this be solved given the information at hand. The wet weight of the sample brick at 65% moisture content is assumed as 15 lbs.

Can you show me how the formula would apply at a few of the data points so that I can see how the volume of the gas is derived?

Also, would it be true that as core gas pressure increased that it would take more and more heat energy to boil off the water in the core? Why?

Thank you

The other info that I left out was that the heat provided from the lab press was a constant 400 degrees F. The wood slurry sample was 12" x 12" x 2" thick - upon being pressed, and as the data indicates, water began to boil off at and above 212 F and a clear increase in gas vapor pressure and increase in core gas temperature occurred. This cycle in the press, the combination of the hydraulic pressure and temperature is supposed to represent a medium density fiberboard process where hydraulic pressure is supposed to dewater some of the sample in the early stage of the cycle, but heat energy does most of the dewatering. I was surprised however to see gas vapor pressures this high from the boil off and partial entrapment of the water vapor. That is all of the info I have.

Hope you can help.


Solution Preview

As we discussed over the message system, you can re-arrange the ideal gas equation as follows:
I have converted T and P into SI units and make the calculation for RT/P as in the attached file.
I then plotted V/n vs. time. This is one of your requirements.

Explanation of V/n graph:
As you can see from the V/n graph, initially V/n start off at high value, then reducing until there's almost a plateau. We can explain that you start off with virtually no vapor, so n is ...

Solution Summary

The use of the ideal gas equation as it applies to boiling water from a wood slurry is discussed.