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Brewer's Spent Grains

Dryers widely used for dehydrating wort sludge both at home and abroad can be generally put into the following four categories, i.e. pneumatic dryer, fluidized-bed dryer, agitating dryer and rotary dryer:
Pneumatic dryer: It can make the wet stock dispersed and suspended in hot air flow, accomplishing heat-matter transfer simultaneously and achieving good quality dehydrated products. When it is used to dry wort sludge, it is hard to remove the moisture in the wet stock due to the bonded water in it, moreover, the drying tube is easily blocked, making the system unsteady in operation. 
Fluidized-bed dryer: when it is running, the direction of airflow is vertical to that of the wet stock, in this case, inlet air temperature should not be set too high for fear that the drying efficiency may be reduced.
Agitating dryer: this type of dryers has the same traits, adopting different mechanism of agitator, through which the heat (the heating medium is usually low-pressure steam, hot oil or hot water)can be transferred to the material. Its major models involve blade dryer, disc dryer and tubular dryer, characterized by high utilization rate of heat, low consumption of gas and low investment in equipment. However, its application scope is restricted to be the boiler, because for this type of dryer, coal, natural gas or fuel cannot be used as the heating source. The maximum heat-exchanging area for tubular dryer is about 700 m2, and its drying capacity is below 2500kg/h.
Rotary dryer: the conventional rotary dryer can meet the needs of drying the coarse particle-formed material, but it is not suitable for high-moisture material and requires improvement on that. Lately, some foreign countries have carried out research and made further progress in this field.

Table 1: Comparison of Several Common Type of Dryers Used for Wet Sludge
equipment Applied medium specification Water removal rate advantages disadvantages
Pneumatic dryer Hot air
<1t/h Short drying time and quick dehydrating speed unsuitable for wet sludge and easy to get blocked
Fluidized-bed dryer Hot air
maximum bed area: 22m2 <3.2t/h Wide range of operation Unsuitable for high-temperature material and easy to get the bed shut down. The temperature of inlet air should not be too high.
Blade dryer steam
maximum area
<1.2t/h Small size and being effective in indirect heat exchange and heat utilization Unsuitable for hot and wet material. The product should be returned with too high moisture content.
Tubular dryer steam
<700 m2 <2.8t/h being effective in indirect heat exchange Hard for maintenance and repair, unsuitable for hot material. The product should be returned with too high moisture content
Conventional rotary dryer Hot air or flue gas
Generally ¢3.4m ~10t/h Being steady in operation Unsuitable for too wet material and the product should be returned with too high moisture content.
Table one shows that all of the dryers listed above have their advantages and disadvantages. Actually, nowadays there are no such equipment specifically used for dealing with hot, wet and great deal of material with low added-value.
Principle of processing wort sludge
The wort sludge has a higher initial moisture-content (generally above 85%), containing cellular water and being poor in fluidity. Besides, its particle distribution is uneven and these particles tend to agglomerate together in its initial stage of drying, hence, its drying process flow should be well designed so as to reduce investment and energy consumption. It suggests as follows: a large amount of free water is removed after separation of liquid-solid, then the required moisture content can be attained through thermally drying and the required particle size can be achieved by smashing the coarse particles. In the meantime, the waste water after separation of liquid-solid will mix with the waste water out of the other sections, flowing into WW treatment plant for further anaerobic and aerobic treatment. After treatment, the water can be discharged when it meets the emission standard.        
This system includes wort sludge cabinet, screw conveyor for screening, screw press, bucket elevator, flight conveyor, coal-fired flue kiln, feeding conveyor, the primary rotary dryer, the secondary vibrating fluidized-bed dryer, crusher, cyclone, bag filter, de-duster, draft fan and blower. The wet wort sludge in storage cabinet is screened by the screw conveyor, thus most of free water in it will be removed. Then the material with 85% of moisture content is pressed by the screw press and the moisture content will drop down to 65%. The semi-dried material is then via the bucket elevator, flight conveyor and feeding conveyor transferred to the primary rotary dryer, where it will be co-currently in contact with clean and high-temperature flue gas of 700℃ from coal-fired flue kiln, being dispersed and intensively dehydrated. After that, it enters the secondary vibrating fluidized-bed dryer, where it will be further dried by means of contacting with the low-temperature off-gas from the primary rotary dryer. The fine product being cooled out of discharging end is the final product, while the coarse product will enter the crusher and the residue will be collected by cyclone and bag filter. The off-gas is vented to atmosphere after being removed by cyclone and filtered by de-duster.
3. Characteristics:
Partial free water in wet sludge is removed by screw conveyor for screening;
The moisture content can be dropped down by screw press from 85% to 65%, reducing the energy consumption in process of drying and improving the efficiency.
The clean and hot flue gas from the coal-fired flue kiln can be as high as 750℃. The heat will be greatly consumed by the primary rotary dryer and the secondary vibrating fluidized-bed dryer and the temperature of tail gas is finally dropped down to 90℃, this way, the heat can be highly utilized in this system, thus greatly improves the heat utilization rate.
The primary rotary dryer adopts a dispersing mechanism, which improves the dehydration ability of beer sludge, shortening the drying time and reducing the size by 1/3 compared to the equipment without this mechanism.
The cyclone plus water-bath de-duster can secure a standard emission.
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