Thermodynamics Evaporation - Help needed

[susuyum]

Hi, 

Recently I was faced with a challenge to find out how long it might take for moisture concentration to reduce from 80% to 30% for soil, under constant sifting/turning . 

Here is the cenario. 

I have a tunnel of 80 meters, width of 4 meters. depth of 2 meters. 
daily, wet soil will be delivered to the front end of the tunnel and everyday, or given a set period there is a machine that mulch/mix and flips the soil backwards by 1 meter. 

The constants are:

Input soil: 10 cubic meters, 80% water content. 

Machine: has blade that mixes the soil and flip it backwards by 1 meter. 

The bed of the tunnel is heated to 50 deg c, can assume the entire soil mass is about 45 deg C

Air temperature is 25 deg C, zero wind speed as it is indoors. 
The tunnel has access to sunlight, but not sure how much it will contribute since i have no way of measuring sunlight's heat or energy generated. 

should look like

|          [Machine]                                                                                                                                         |
|~~~////~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
|___////_______________SOIL_________________________________________________________________|

Would like to know how long will it take 'distance' for the soil's moisture content to drop to 30%.

I've tried my best searching for a correct formula to use, but couldn't find one for soil, and then using water evap formula for swimming pool has got me very strange results. 

 

Please help and thank you in advance. 

Stv

[studiot]

Water in soil has an additional complication due to the what is known as the pore pressure.

This is a measure of the interstitial and intermolecular forces for different soils, clay for instance has a high cohesion which holds the water and slows the drying process compared to a non cohesive soil such as sand.

So the first thing to do is to identify the soil type and measure its cohesion, or better work on a range of soil cohesions for your application.

I would also look at the websites of manufacturers of grain dryers who face a similar problem.

Edited February 23, 2018 by studiot

[Bender]

Any relevant formula I can throw at you is going to produce very wild guesses at best.

The air in between the soil particles will probably be saturated quite quickly, and natural convection will take a long time to evacuate a significant amount of water. You could do a test with a small amount of soil and see how deep under the surface the soil has dried. I assume that under that dept, there will be hardly any change in humidity at all.

You could at the very least make an energy estimate. How much water do you need to evaporate, and what is the maximum power you can put into heating. At the very least, that should give you a lower boundary for the total length.

But for any reliable estimates, I would advice a test with one heap of soil which you can mix every day and track the humidity. 

Alternatively, if there are existing facilities nearby: visit and copy/extrapolate. 

[susuyum]

Studiot / Bender, Thank you for your help and ideas. 

I understand the problem I've got on hand is a very complicated one, and is probably a little time consuming to calculate instead of simply experiment to get some bases. 

Below I've included a picture of what such facility looks like. Except the one below the soil depth is 1 meter instead of 2. 

Much appreciated. 

[Frank]

Other factors are initial air humidity and a heated bed creates a convective air flow - otherwise where does the water vapour go?

I guess this knowledge is needed to compare to other methods.  Another method came to mind: Use a geothermal heat pump to heat the soil and cool/dry the incoming air then the dry air absorbs moisture as it is blown/drawn across the soil and out again, assuming electricity is used for heating.

Interesting company, biowon, they even talk about Mars soil potential, though wet soil isn't likely a Mars issue.

 

[Bender]

20 hours ago, susuyum said:

Below I've included a picture of what such facility looks like. Except the one below the soil depth is 1 meter instead of 2. 

Doubling the depth is probably going to more than double the required length, since evaporation will only be effective at the surface.