Drying Technology
In the process of sewage treatment, most of the pollutants are converted into sludge, so the sludge contains various pollutants with a wide coverage, including various heavy metals, traces of highly toxic organics (PCBs, AOX, etc.), and a large number of bacteria, virosomes, and parasite eggs and other pathogenic microorganisms. If they are not properly treated, it will cause environmental sanitation and pollution problems, and easily cause secondary pollution. In recent years, sewage drying technology has developed rapidly, and several important sludge drying technologies are introduced below.
Types of Sludge Drying Equipment
According to the contact method of heat medium and sludge, it can be divided into:
1. Direct heating type: the hot air generated in the combustion chamber is directly mixed with the sludge, so that the sludge can be heated, the water can evaporate and the dry sludge product is finally obtained, which is the application of convection drying technology;
2. Indirect heating type: the heat generated by the combustion furnace is transferred through the steam and hot oil medium to the heater wall, so that the wet sludge on the other side of the wall is heated and the moisture evaporates to be removed. It is the application of conductive drying technology ；
3. "Direct-indirect" combined drying: it is a combination of "convection-conduction technology".
According to the form of equipment, it is divided into:
Rotary drum type, turntable type, belt type, spiral type, centrifugal dryer, spray-type multi-effect evaporator, fluidized bed, multiple coil type, film type, paddle type and many other forms.
According to the feeding method of drying equipment and product form, it can be roughly divided into two categories:
One is to use a dry material back-mixing system. The wet sludge is mixed with a certain proportion of dry sludge before feeding, and then enters the dryer. The product is spherical particles, which is a process that combines drying and granulation; the other is the direct feed of wet sludge, and the products are mostly in powder form
Working Principle and Process Flow
1. Direct heating drum drying technology
The working principle is: the dehydrated sludge enters the mixer from the sludge funnel, and the part of the dried sludge is fully mixed in proportion to make the solid content of the dry and wet mixed sludge reach 50%-60%, and then passed through the spiral The conveyor is transported to the three-channel drum dryer.
The hot air flow entering the drum and the same end is 1.2-1.3m/s and the temperature is about 700℃. The hot air is mixed and heated. After about 25min of treatment, the dried sludge is conveyed by a screw with a metering device. The machine is sent to the separator, where the hot and humid gas discharged from the dryer is collected for thermal recycling, and the polluted malodorous gas is sent to the biological filter for treatment and meets the environmental protection requirements. The dry gas discharged from the separator The particle size of the sludge can be controlled, and then the sludge particles that meet the requirements are sent to the storage bin for processing through the filter.
The dryness of the dried sludge reaches 92% or higher. The diameter of the dried sludge particles can be controlled within 1-4mm, which mainly considers the possibility of using dried sludge as fertilizer or landscaping. The fine dry sludge is sent to the mixer to mix with the wet sludge and sent to the drum dryer. The burner used to heat the drum dryer can use biogas, natural gas or hot oil as fuel.
The separator separates the dried sludge and water vapor. The water vapor carries almost all the heat consumed when the sludge is dried, and this part of the heat needs to be fully recycled. Therefore, the water vapor has to pass through the condenser. The cooling water inlet temperature of the condenser is 20°C, and the outlet water temperature is 55°C. The cooled gas is sent to the biofilter for treatment and is discharged after it reaches the emission standard.
The characteristics of the drying system: operating in an oxygen-free environment, no dust is generated, the dried sludge is granular, the particle size can be controlled, and the gas recycling design reduces the cost of exhaust gas treatment.
2. Indirect heating drum drying technology
The dewatered sludge is transported to the feed hopper of the dryer, and sent to the dryer via a screw conveyor, which can be controlled by frequency conversion and quantitatively conveyed. The dryer is composed of a rotating drum and a wing screw. The rotating drum is heated by a combustion furnace. The maximum rotating speed of the rotating drum is 1.5r/min. The vane screw transfers heat through circulating hot oil, and the maximum speed is 0.5 r/min. The drum and the vane screw rotate in the same direction or in the opposite direction, and the sludge can be continuously moved forward for drying. The drum is distributed in three combustion furnace temperature zones along the length direction, which are 370℃, 340℃ and 85℃ respectively. The temperature of the hot oil in the vane screw is 315°C. After the drum is exhausted, the internal pressure is negative, and water vapor and dust cannot escape. The sludge is gradually dried and ground into granules by the drum and the wing screw and heated, and is sent to the storage bin by the dry sludge screw conveyor through the S-shaped air check valve in the low temperature area at the back of the drum. The water vapor evaporated from the sludge is sent to the condensation and washing adsorption system through the system exhaust fan. The characteristics of the drying system are: the process is simple, the dryness of the sludge can be controlled, and the end product of the dryer is powder.
3. Centrifugal drying technology (dehydration and drying integrated machine)
The thin sludge enters the centrifugal dryer from the thickening tank or digestion tank. The centrifuge in the dryer dewaters the sludge. After mechanical centrifugal dewatering, the sludge is discharged from the centrifuge discharge port in a fine powder form at high speed and high heat. The air is introduced into the centrifugal dryer in an appropriate way, meets the fine powdery sludge and drys it to a solid content of about 80% in the shortest time. The dried sludge particles are pneumatically discharged from the dryer at a temperature of 70°C, and enter the cyclone separator together with the wet waste gas for separation. Part of the wet waste gas enters the scrubbing tower, where most of the moisture in the wet waste gas is condensed out, and the purified waste gas leaves the scrubbing tower at a temperature of 40°C. The characteristics of the drying system are: the process is simple, and the sludge dewatering machine and the storage, transportation and transportation devices from the dewatering machine to the drying machine are omitted.
4. Indirect multi-tray drying technology (pearl process)
Its working principle is: the sludge after mechanical dewatering (solid content 25%-30%) is sent to the sludge buffer silo, and then transported to the coating machine through the sludge pump, and the dry sludge is recirculated in the coating machine The sludge particles are mixed with the imported dewatered sludge. The outer layer of the dry particle core is coated with a layer of wet sludge to form particles. This coating process is very important. The inner core is dry (solid content> 90%), and the outer layer is A layer of wet sludge, the particles coated with wet sludge are sent to the hard granulator (multi-disc dryer), poured into the upper part of the granulator, and evenly scattered on the top disc. By the action of the rake on the rake arm connected with the central rotating main shaft, the sludge particles make a circular motion on the upper disc. The sludge particles move from the upper disc of the granulator to the bottom disc of the granulator by gravity. When the particles move on the disc, they directly contact the heating surface and dry. The sludge particles gradually increase, similar to the formation process of pearls in mussels, and finally form solid particles, so it is also called pearl technology. The temperature of the dried particles is 90°C and the particle size is 14mm. After leaving the dryer, they are sent up to the separation hopper by a bucket elevator, and a part is separated and recycled back to the coating machine. At the same time, the remaining particles enter the cooler to cool to 40° Sent into the particle storage bin. The energy required for the sludge drying process is transferred by the hot oil. The hot oil with a temperature of 260～230℃ circulates in the hollow disc in the dryer. The steam discharged from the dryer is condensed at close to 115℃, and then passed through the heat exchanger. The temperature of the hot water after condensation is 50-60°C. This indirect multi-disc dryer is also called a granulator, with a vertical arrangement of multi-stage distribution and indirect heating. Features: The oxygen concentration in the drying and granulation process is less than 2%, which avoids the danger of fire and explosion. The particles are round, solid, dust-free and uniform, and have high calorific value. They can be used as fuel. After being condensed and washed with water, the exhaust gas is sent back to the combustion furnace to completely decompose the odor-producing compounds, so the exhaust gas can meet very strict requirements. Emission standards.
5. Fluidized bed sludge drying technology
Its working principle is: the dewatered sludge is sent to the sludge metering storage bin, and then the sludge pump is used to send the sludge to the inlet of the fluidized bed sludge dryer and the sludge is distributed. The fluidized bed sludge dryer is basically composed of three parts from bottom to top. At the bottom of the dryer is a wind box, which is used to distribute the circulating gas to different areas of the fluidized bed device. A special gas is installed at the bottom. Distributing plate, used to distribute inert fluidizing gas. In the middle section, the heat used to evaporate the water will be sent into the fluidized bed by heating the hot oil. The uppermost part is the suction hood, which is used to separate the fluidized dry particles from the circulating gas, and the circulating gas leaves the dryer with the sludge particles and evaporated moisture. In the dryer, the drying temperature is 85°C, and the sludge particles produced are fluidized by the circulating gas and mixed vigorously. Because the fluidized bed relies on its own heat capacity, long residence time and large quantity of products, even if the quality of the feed or moisture is somewhat fluctuating, the drying can be ensured evenly, and the sludge particles and dust are carried out by the circulating gas. The sludge particles are sent to the cooler through the rotary air lock valve, condensed to less than 40℃, and sent to the product silo through the conveyor. The dust and sludge fine particles are separated from the fluidized gas in the cyclone separator, and the dust and sludge fine particles are transported from the ash bin to the spiral mixer through the metering screw conveyor. There, the dust is mixed with the dewatered sludge and sent back to the fluidized bed dryer through a screw conveyor. The fluidizing gas of the dryer system and the cooler system is maintained in a closed gas loop. The circulating gas takes the sludge fine particles and evaporated moisture away from the fluidized bed dryer. The sludge fine particles are separated in a cyclone separator, and the evaporated water is condensed in a condensing scrubber by direct countercurrent spraying of water. The evaporated water and other circulating gas are cooled from around 85°C to 60°C, and then condensed. The condensed water leaves the circulating gas and flows back to the sewage treatment area. The clean and cooled fluidized gas in the condenser returns to the dryer for drying. The chemical sludge is cooled to below 40°C by the cooling circuit gas. The characteristics of the drying system are: no return material system, indirect heating, and the dryer itself has no moving parts, so there is almost no need for maintenance, but the particle size of the dried particles cannot be controlled.
In addition to the above, there are flash dryers, screw dryers, film dryers, spray dryers, multi-effect evaporators, microwave dryers, belt dryers, multi-bed dryers, etc., the same type Drying equipment also has its own characteristics in different manufacturers. Among them, Shincci, which is ranked Top1 among the Chinese manufacturers of dryers that are the most advanced in technology and widely used in international projects, their dryers are low-temperature evaporative dryers, from the overall applicability of the machine, service life and after-sales service The details of spare parts, wearing parts and equipment optimization are the highlights of his brand.
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