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New Mixing Solutions for Bio-Refineries

Stirred reactor cascades unite the advantages of tube reactors with stirred tank reactors during continuous operation. Several alternatives when designing the cascade permit more viscosity control.

The energy supply of the future will be increasingly based on the use of biomass from organic wastes. Biofuels of the second generation, such as bioethanol, biodiesel or plant oil, are tapping into new sources, such as liquid manure or straw, and therefore no longer directly compete with valuable foodstuffs as was the case in the first generation.

In addition, efforts are being made to manufacture so-called platform chemicals from biomass. These can then be used in the production of plastics, as an example. Numerous demonstration facilities are already in operation; industrial manufacture is only a matter of time.

For an environmentally sensitive, as well as economically efficient plant operation, the EKATO Group offers innovative solutions for new process engineering methods, which have been developed together with the customer based on laboratory and pilot plant tests, and can be reliably scaled up to production capacity.

The digestion of the biological raw material that is aided by micro-organisms in the digester, as well as the biological metabolism in the fermenter, requires certain preconditions in order to operate efficiently. These are influenced by the end product, the raw materials used, as well as the micro-organisms and enzymes employed.

Critical influencing factors are temperature control, avoidance of sedimentation, supply of oxygen for certain micro-organisms and the homogeneity of the vessel contents that fulfill the basic mixing task.  

Tube Reactor or Continuously Stirred Tank Reactor?

With the so-called tube reactor, the vessel is much higher than its diameter. Mixing in the flow direction does not take place; the composition over the cross section is constant. Everything that is dosed at the same time into the reactor also leaves it together.

This simplifies an optimization of the vessel volume. In contrast, however, the lack of back-mixing in the flow direction increases the risk of a non-homgenous mix, and it is difficult to avoid the sedimentation of solids or guarantee efficient gassing in the presence of solids.

The alternative is a continuously operated stirred reactor in which the feed flow is immediately and completely mixed, and the output flow has the same composition as the vessel contents. A certain partial flow can leave the vessel without having taken part in the digestive process. Solids can be suspended without difficulty in stirred tanks, a homogeneous mix is achieved, and reactant gases are evenly distributed, however, the residence time for material that was dosed simultaneously is stretched over a larger time interval.

A Cascade of Stirred Tanks Reduces Back-Mixing

An entire series of several stirred tanks in continuous operation maintain its original advantages while simultaneously reducing back-mixing.

At the same time, the residence time behavior of a cascade becomes more and more similar to that of a tube reactor with the same volume when the quantity of vessels in the cascade are increased. Due to the fact that the solids content and viscosity often change significantly during the process in a bio-refinery, it is possible with the cascade principle to intelligently combine the central impeller and close-to-the-wall impeller with one another.

For example, less expensive central impellers can be used — except in the first vessel — as the viscosity continues to drop from vessel to vessel as a result of the progressive hydrolysis of the basic raw material. As a rule, the viscosity is already low enough in the second vessel to operate without the cost-intensive close-to-the-wall impeller.

Even replacing the first vessel with two intermittently operated vessels can additionally reduce viscosity if the design takes that into consideration, so that it is possible to use central agitators.

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