3 replies to this topic

[randy]

10% by weight water is mixed in fuel oil and used in a direct fired rotary dryer. The product being dried has initial and final moisture levels of 3% and 0.5 % , respectively. The product temperature is introduced into the dryer at ambient temperature and exits at 140 deg C. My question is : does adding water to the fuel affect the drying process in any way ?
Randy

[sgkim]

Hello randy..
The water added to fuel oil as atomizing steam is also used for increacing H/C ratio especially for heavy oils which are lower in H/C ratios than light hydrocarbons. So steam provides a good complete combustion state. Even if steam is added, the flame temperature would be still high, say, more than 750 deg C. At atmospheric pressures and such high temperaures of solids around 140 deg C, the most part of water may be vaporized from the solids.

You must have to increase the holding time of the solids in the rotary drum dryer if the water content of the solids is high at the outlet. The evaporation rate of water from the the solids will be decreased as the size of the solids increases. I don's think you really use water instead of steam. Steam will save the fuel consumption.

[Adriaan]

1.) Adding water to off-gas can improve the heat transfer, I don't think it makes much difference HOW you add the water (be it as steam or by mixing water with the fuel).

2.) Added water increases off-gas volume significantly; every volume unit of water produces 3600 volume units of steam under atmospheric conditions. The increased volume can help by changing the flow of the off-gas (more turbulence).

3.) Adding water to the fuel reduces the flame temperature (and thus the off-gas temperature), this might simply be to limit the temperature (to avoid burning the product). It can also help prevent the formation of certain components like nitrous oxides in the combustion.

Drying products .... well if you dry them fully you can end up losing money! Coffee companies roast coffee beans and in the process the beans are dried out. To stop the roasting water sprays are used; they are used is such a way as to INCREASE the moisture content because that means they can sell some water at coffee bean prices

Everything Adriaan said is correct.

The driving force in drying is the water concentration in the solid vs the equilibrium water concentration in the vapor phase, so from that stand point adding water to the oil MIGHT be slowing down the drying a little bit. But there might be other considerations - improved heat transfer from more turbulence, wanting to limit the drying of the product, etc. It is certainly true that adding water to the oil will be limiting the flame temperature. Occaisionally that is used as a smoke suppression, and conceptually for Nitrogen Oxides (NOx) suppresion.