Agitators Figure

Mixing by filling in of new substrate, by thermal convection flow, and by raising of gas bubbles is mostly not sufficient for agricultural biogas plants. Only small plants can be operated without agitators.

The mixing operation is done mechanically by devices moving in the bioreactor, hydraulically by pumps located outside of the tank, or pneumatically either by pumping in biogas or by using the autonomously generated gas- pressure for pump work, potentially with two-component nozzles (Table 4.4) .49'

Excessive mixing increases power consumption, deteriorates the energy balance, and disturbs the microorganisms, thus ultimately decreasing the gas output. Consequently, various different mixing strategies have been developed:

• Slow - moving agitators with arms and paddles reaching the whole of the fermentation space, with low energy consumption and operating continuously.

• Medium speed agitators running intermittently or continuously with modest energy consumption.

• High speed, intensely operating agitators with high energy consumption, which are switched on repeatedly but only for a short time, e.g., every three hours for 15 min during a day.

Screw agitators with submerged motor adjustable vertically, horizontally, and with a changeable tilt angle are mainly used.

The required agitator intervals and times have to be determined individually for each biogas installation. Shortly after installation, mixing should be performed more frequently and for longer times as a precaution.

Paddelgigant

Figure 4.8 Agitators.

Axial agitators: steplessly adjustable in its height (top left) Casting for lifting the agitator out of the reactor (top right); Adjustable shaft mixer driving and agitator of 1.2 m diameter (middle left); Ruhrgigant FRfor biogas-fermenter (middle right); Paddelgigant™ (bottom).

Figure 4.8 Agitators.

Axial agitators: steplessly adjustable in its height (top left) Casting for lifting the agitator out of the reactor (top right); Adjustable shaft mixer driving and agitator of 1.2 m diameter (middle left); Ruhrgigant FRfor biogas-fermenter (middle right); Paddelgigant™ (bottom).

Table 4.4 Application frequency as a percentage of all plants with different agitators in Europe.

Type

Application frequency

Remarks

Submerged

44%

These agitators are vertically installed in vertical bioreactors,

motor

where temperatures are below 70°C. The motor (2.5-

propeller

25 kW) is liquid-tight encapsulated. The propeller agitator

agitators

with 2 or 3 blades is directly fixed to the motor shaft. In vertical bioreactors with a much bigger diameter than the height, they are installed horizontally through the wall.

Axial agitators,

16%

Axial agitators are installed to produce a torus-like vortex

shaft mixer

with an upward stream along the wall. Through this liquid stream it is possible to destroy permanently the floating cover by sucking it to the bottom in the middle of the reactor. The speed of rotation of the electric motor must be low.

The shaft mixier with its motor at the upper end of the shaft can be vertically and horizontally positioned in the reactor. Therefore the opening in the cover of the bioreactor is flexible. Additionally to the agitator itself, cutting devices can be installed to cut fibrous material.

Paddle

10%

Paddle agitators are best suited to generate horizontal plug

agitators

flow in a horizontal cylinder. Across the flow direction they balance differences in concentrations and support the heat exchange.

Agitators50) with four inclined paddles is designed to be used in vertical bioreactors with substrates of high concentration of fibrous material like grass silage. The long paddles hinder the formation of a scum. They rotate slowly and therefore save energy.

Pneumatically

ca. 12%

A part of the biogas is cleaned and then circulated by a

forced

compressor into the reactor, so that gas bubbles ascend in

rotation

the substrate, and it is agitated.

Other

18%

Some agitators, called "Grindel agitators", have a vertical

agitators

shaft and are installed eccentrically. They form a vertical vortex, which is always broken at the reactor walls. Grindel agitators are not suitable to destroy scum and a sinking layer.

As a rule, a brief but intense mixing is better than a prolonged but weak one. In practice, a mixing frequency of 1-10 times a day and mixing duration of 560 min has proven to be useful.51'

All electrical devices in a continuously run bioreactor have to be explosion proof, suitable for zone 2. In discontinuously run bioreactors the requirements explosion proof for zone 1 have to be met.

Submerged Fermenter
Figure 4.9 Bioreactor without rotating agitator.

Submerged motor agitators or submerged motor pumps have to at least conform to protection class IP 68 according to the European standard EN 60529 and can only be run when submerged. An appropriate standard operating procedure is required.

Installations with artificial leveling do not require a rotating agitator (Figure 4.9). The bioreactor52),53) mainly consists of a cylindrical container with a central, cylin-dro-conical tube C and overflow D.

Initially the bioreactor is filled up to the upper brim of the overflows. The gas valves are closed at first. As a consequence of the gas development, the gas accumulates in compartment A below the conical insert and the liquid is pushed up through the cylindrical insert C.

A part of the liquid flows into the overflow ring D. The gas in compartment B is compressed. Once the fermentation time is completed, fresh substrate is pumped into the cylindrical container C. The same quantity of fermented substrate is discharged from the overflow ring.

After the refilling operation, the gas in compartment A is withdrawn into compartment B through valves, causing the liquid level to fall and the newly inserted liquid to be pushed into the bottom part of the bioreactor. Thus, an intensive mixing of the reactor compartment takes place and the floating layer is destroyed.

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Responses

  • robinia galbassi
    How does an agitator work biogas?
    7 years ago

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