Sand Drying 1

[drdust]

Client wishes to convert a totating kiln, counterflow grain dryer to a sand dryer. The sand is aproximately 5% moisture, and they want to dry to less than 1% moisture. How do I determine the heat required and the airflow rate required through the dryer.

they wish to dry 90 tonnes/hr.
the dryer is 1.5m in diameter and 8 m long.

Do not know the rpm available.

Do know they have a 18,000,000 btuh oil fired burner.

 

[MintJulep]

Not enough information here to solve the problem.

Need to know the entering temperature of the wet sand, and the temperature and absolute humidity of the entering air.

What you need to do is raise the vapor pressure of the water in the sand above the vapor pressure of the water in the air at the exit of the drier.

 

[drdust]

Unfortunately I do not know these two items. I believe the sand will be in the 15 C range and the air from 10 C to 30 C and the humidity of the air at 50 to 80% depending upon the day. The design conditions according to ASHRAE should be maximum 31Cdb and 19 Cwb with a mean daily range of 14 C. Design wet bulb 21C. This gives me the vapour pressure range for the air.

How do I determine the vapour pressure of the water in the sand?

Thanks for your help.

 

[MintJulep]

Fortunately, wet sand is simply a mixture of dry sand and water. Each sand grain gets wet on the outside, but doesn't absorb any water. So the vapor pressure of the water in the sand is simply the vapor pressure of water at the wet sand's temperature and your local atmospheric pressure. You can look this up easily enough, here

http://dbhs.wvusd.k12.ca.us/GasLaw/Vapor-Pressure-Data.html

for example. (I assume that this table is for standard sea level pressure).

This is a bit of a simplification, but essentially evaporation is driven by the difference in vapor pressures. The higher the difference, the faster the evaporation rate. Having air with a relative humidity of 80% makes the job a lot harder, as it just can't hold much more water.

The partial pressure of the water in the air is a function of temperaure and humidity. If you heat the air the absolute humidity (mass of water per mass of air) remains constant, but the relative humidity decreases. Unfortunately the vapor pressure of the water in the air also increases.

Also, the hotter the air, the hotter the sand needs to get to maintain a decent driving differential. Of course the air heats the sand as they pass. Fortunately you can't damage sand by overheating it.

Its really best if you can take some of the water out of the air before it enters the drier.

I hope this at least gets your thinking headed in the right direction.

Of cousrs you could always take the brute force approach. Just heat the sand above 100C. But this is hardly efficient, and not nearly as much fun.

I found these two web sites interesting.

http://van.hep.uiuc.edu/van/qa/sect...Evaporating_and_Condensing/20020321122324.htm

http://www.pdh-odp.co.uk/GasLaws.htm

 

[hacksaw]

for get it.

you don't have enough residence time and the moc's of most grain driers are incompatible with sand.

 

[wms]

You could consider a recirculating system, being fed in excess of 90TPH, and reclaiming at 90TPH.