What is sludge drying operation

Introduction of sludge drying:

sludge drying is to remove the moisture in the sludge. There are two main processes for the removal of moisture: 

(1) Evaporation process: the water on the surface of the material is vaporized. Since the water vapor on the surface of the material is lower than the partial pressure of water vapor in the medium (gas), the water moves from the surface of the material into the medium.

(2) Diffusion process: it is a mass transfer process closely related to vaporization. When the water on the surface of the material is evaporated, the humidity on the surface of the material is lower than the humidity inside the material. At this time, the driving force of heat is required to transfer the water from the inside to the surface.

The continuous and alternate execution of the above two processes basically reflects the drying mechanism. Drying is accomplished by two complementary and parallel processes of surface water vaporization and internal water diffusion. Generally speaking, the diffusion rate of water decreases with the increase of the dryness of the sludge particles, while the vaporization rate of surface water increases. increased with increasing dryness. Since the diffusion speed is mainly driven by thermal energy, for the thermal convection system, the dryer generally adopts the co-current process, and the thermal energy supply of most processes is gradually reduced, which results in the drying speed of the high dryness product in the second half. reduce. For the heat transfer system, when the surface moisture content of the sludge decreases, the heat exchange efficiency drops rapidly, so a larger heat exchange surface area is necessary to complete the evaporation of water.

The Purpose of sludge drying:

The difference between so-called drying and semi-drying in sludge drying lies in the moisture content of the dried product. 'Fully drying' refers to the type with higher solid content, such as more than 85% solid content; while semi-drying mainly refers to the type with solid content between 50 and 65%.

If the purpose of drying is sanitation, the sludge must be dried to a higher solid content, up to 90% or more. At this time, the moisture contained in the sludge is much lower than the average air humidity at ambient temperature, and the return When exposed to the environment, it will gradually absorb moisture.

If the purpose of drying is only to reduce the amount, different solid content requirements will be generated. When the wet mud with a solid content of 20% is dried to 90% or to 60%, the reduction ratios are 78% and 67%, respectively, with a difference of only 11 percentage points. Depending on the purpose of the treatment, different solids contents are actually required. For example, in landfill, the average solid content of landfill waste is less than 60%, and it is economically meaningless to require sludge to reach 90% solid content.

Therefore, it should be an economical and reasonable requirement to dry the sludge to the equilibrium and stable humidity in the treatment environment, that is, the partial pressure of water vapor in the surrounding air and the water vapor pressure on the surface of the material reach equilibrium.

The status of sludge drying process:

Some sludge drying processes can treat wet sludge to a solid content of 50-65%, and the treatment capacity at this time is significantly higher than that in full drying. There are two reasons for this:

First, for drying systems, drying time determines the throughput of the dryer. When the moisture content of the material is high (so-called semi-drying), the time for evaporating the same amount of water is less than when the moisture content is high (so-called full-drying), and a higher processing capacity can be achieved per unit processing time.

Secondly, the rate of water loss of sludge under different drying conditions is different. When the moisture content is high, the water loss rate is high, and vice versa. Most drying processes take 20 to 30 minutes to dry sludge from 20% solids to 90% solids.

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