Application of rake dryers in organic waste treatment

Rake dryers are effective at processing organic waste, also known as wet waste, which refers to the organic components of municipal solid waste. A large portion of this consists of kitchen scraps and restaurant food waste. Therefore, proper handling of this waste is crucial for environmental protection. The process involves: first, removing non-biodegradable components from the food waste; then, after crushing and hydraulic separation, forming an organic emulsion, which is then fed into a fermentation tank with a specific ratio of auxiliary materials and fermentation bacteria for aerobic fermentation, resulting in a solid-liquid mixture. This mixture is then sent to a rake dryer for drying, followed by granulation, drying, and bagging processes to produce high-quality bio-organic compound fertilizer.

The rake dryer utilizes its intermittent operation characteristic, combining fermentation and drying within the same equipment. The organic waste is thoroughly mixed with the bacteria and bacterial beds, and fermented under suitable temperature through the action of enzymes. This process decomposes the carbohydrates, fats, and proteins in the organic waste into carbon dioxide, ammonia, and water vapor. After deodorization by high-temperature catalyst, colorless and odorless gases are discharged, resulting in a weight reduction of over 90%. The residue is non-toxic and is an excellent organic fertilizer.

1. Characteristics of the Rake Dryer

(1) Simple structure, convenient operation, long service life, and stable and reliable performance.
(2) Strong applicability, wide application, and fast drying speed: Because the rake dryer utilizes jacket heating and high vacuum exhaust, it is suitable for almost all materials of different properties and states, especially for explosive, easily oxidized, and paste-like materials.
(3) High product quality: During the drying process, the rake teeth rotate continuously in both directions, stirring the dried material evenly, avoiding overheating, and allowing moisture to easily escape.
(4) Low steam consumption.
(5) Easy to operate, recoverable volatile gases, and low environmental pollution.
(6) Fine product particle size, eliminating the need for crushing procedures. 2. Main Components of a Rake Dryer

(1) Shell; (2) Rake teeth; (3) Discharge device; (4) Feeding device;

(5) Crushing rod; (6) Sealing device; (7) Stirring shaft; (8) Transmission device

3. Working Principle

The material to be dried is added from the top center of the shell. Under the stirring action of the constantly rotating rake teeth, the material is moved axially back and forth, and the surface in contact with the inner wall of the shell is constantly renewed. Simultaneously, it is indirectly heated by steam. Under the uniform stirring action of the rake teeth and the crushing action of the crushing rod, the surface of the material is constantly renewed and enlarged, making it easier for moisture to be discharged. The vaporized moisture and other exhaust gases are treated by a dry or wet dust collector and condenser to remove dust and potentially condensable gases. The non-condensable gases that meet the emission standards are extracted and released by a vacuum pump. 4. Thermal Calculation

Customer-provided material parameters: 2 tons of organic waste with a moisture content of 65%–80%, plus 1 ton of rice bran or wheat bran with a moisture content of 10%–15%, totaling 3 tons of raw materials. The overall moisture content reaches 50%–55%, suitable for bio-fermentation. After fermentation and drying, the material moisture content is 10%. Processing capacity is 3 tons/day, equivalent to 0.143 tons/hour (calculated based on 24 hours of daily operation, considering feeding and discharging time). The drying heat source is saturated steam at 0.1 MPa (gauge pressure).

5. Usage Results

Our designed rake dryer has been in use at a company for one year. Each unit has an annual processing capacity of 1000 tons, consuming nearly 20% less steam than traditional methods such as convection drying. By eliminating the need for fermentation equipment, labor and conveying equipment are saved, resulting in a reduction of nearly half of the electricity. The expected energy-saving requirements have been achieved. 6. Application Prospects

Rake dryers are mostly operated under slight negative pressure or vacuum. During the drying process, moisture vaporizes at low temperatures, achieving low-temperature drying. This is advantageous for drying heat-sensitive materials in certain pharmaceuticals, foods, and agricultural products. For example, some components of liquid sugar turn brown above 70°C, reducing the product's commercial value; certain catalysts change their chemical properties above their operating temperature; and proteins denature at high temperatures, altering the nutritional composition of the material. Furthermore, low-temperature drying optimizes heat energy utilization, thereby reducing production costs, improving product quality, reducing labor intensity, and protecting the environment. Therefore, since its successful development, the rake dryer has been widely used in the pharmaceutical, chemical, dye, pesticide, and food industries.